@Phill
uhhh very nice... PanzerGrenadier PM's... Don't know if you guys are familar with German WWII (aswell todays) warfare.
In the time of early and- midwar the Wehrmacht used their mounted-units-tactica. Which is basicly to have a troop of "PanzerGrenadiere" assaulting next to their tanks to prevent the enemy to kill the tanks in CC and to clear out hideouts. Was very brutal in City fights, because of the tanks lack of manouverability.
And of course the troops rode in their SdKfz250 "Hanomags" I think this could be combined with a Plaguespear-tactica or a with two Preds defended by 1 unit each and a few extra rhino-mounted units.

@Oldschool
Robot Boy Toy are very limited in their lists-choice Imo, so Destroyer and normal warriors is always an enemy to fear, even for DG. The 'crons have their WWBB-rule which is pretty much like FnP. But they have large units in the worst case... so it's very hard to kill 10+ of them in turn to prevent them from getting back (they only can comeback if a 'cron unit is nearby).
Destroyers have S6 weopons right? and Heavies have S8? hmm two units outflanking you... I'll better run off the table... I hate those destroyers
Those suckers have T4 and are pretty similar to marines, so Plasma and even more plasma is your friend. SM AssaultCannons would come pretty handy... sadly we have none... HB's will maw some down, but are not strong enough to insta-kill them. Vindicators are possibly usefull.
I should do a testgame with my brother... and hijack is codex

ok, got another response from a general for you:

Our gaming group took a Epi Tally List with three Death Guard Marines Armies to Adepticon 09. We took best Heretical Team and the only way to bring down Plague Marines is power weapons, and force multiple saves. A 10 man squad of plague marines can handle 60 + Ork Boyz even with a power claw nobz. The only way to beat them like I said is forcing the Plague Marine to play the odds. 3+ 4+ saves they are bound to fail some from mass bolters and heavy bolters. The only units I feared was units with power weapons that went off at IN, Plasma Cannons and CombiBolter Sternguard (Melta).

If you are going to play a Plague Marine Army I would go all or nothing.
HQ: 2 Nurgle Daemon Princes with wings and warp time

Elite: None

Fast Attack: None

Heavy Support: 2 or 3 pairs of Oblits

Troops: A many 10 man squads of Plague Marines with melta and flamer, a Plague Champion with PF all loaded into a bare bones Rhino.

If it Orks your are facing just use your Oblits firing plasma cannons and bolters from the rhinos. Tie up large blocks of troops with the DPs or use them both to go warboss or loota hunting. Do not get out of the Rhinos will the last second and double tap and flame if you know that you can weather the charge from the boyz. If not, flame, bolt pistol and charge. Do not let them get get the charge bonus on your Plague Marines. Also use your Oblits on the Nob Bikers. First turn they will turbo 24 and will get a 3+ save. Melta will kill each Nob Biker outright and the ork player will have to remove a whole, unwounded model first. No stacking wounds and hopefully will kill of a Power Klaw or two.

Other Marines, it is just a slug fest and your FNP save will help you get through it all. If they have power weapons then just try avoid CC till you have a 2:1 ratio.

Tau, charge into them. You go Simo. Use the DPs and Oblits on the Crisis Suits. Railguns there is nothing to do against them but they are only 1 shot so one dead Plague Marine and they are fearless.

The new IG Codex: Prey to the PlagueFather because it is template hell on Plague marines. But they still get FNP.

Plague Marines will eat Robot Boy Toy. No worries there. Ignore the Monoliths, just go after the warriors and phase them. Thier WBB rolls is not as good as your FNP so grind them into the dirt with your pus covered boot.

Smelldar against Plague Marines. Against most the FNP will save you.

Daemons just force them to make saves against your bolters that is all I can tell you. Crappy 5+ saves and Fearless saves will cause them to fail hopfully in time. Other than that don't get out of the rhinos and double tap. Also, Flamers will do the best agaisnt them since their saves are crappy.

Witch and Daemon Hunters are not an issue. If they do not DS on you or use an Orbital Stike then you really have nothing to worry about. Use marines to take out the Inqusitors and the DPs to hunt the Sisters or GKs. The oblits firing plasma cannons should be able to handle the GK termies. Sister Repenta with a prest, HA! Move up a squad on both sides if possible, get out, flame and double tap. They will fall and you will have IN when they charge since thier weapons go at an IN 1.

I hope this helps out. Playing many great players at Adepticon and owning many different armies has taught me alot. For the points Plague Marines are where it is at IMHO.

another:
There is still one unit that we all have forgotten that can harrass and kill Plague Marine units without being a uber expensive HQ or long range template armor. That is Tau Crisis Suit Teams. I always carry one unit of Anti-Tank and one Anti-Horde Units. Now you can configure these to kill Plague Marines and they are cheep compaired to a space puppy HQ unit I saw that was said to be able to take on a unit of plague marines or even most HQ Units. For alittle over 200 points it is three Crisis Suits with a Tau Sgt. All have multi trackers Fusion Blasters and a Plasma Rifle. Give the Sgt the upgrade to target two units at the same time and you have a Plague Marine Killing unit which will bypass the FNP since all their AP is 2 and 1. So deep strike within 12" of the rhino, allow the Rails to pop the tank and then target the juicy bits that spill out. If the railguns fail then the Tau Sgt can try first with his Fusion Blaster and Plasma Rifle and the squad target something else. Also this unit is great against DPs, Greater Daemons and Oblits since all thier fire which will wound on a 2, 3 or 4 will force them to take their 5+ Inv Save.

The other type of Crisis Suit that will be hell on Plague Marines is the Anit-Horde all with Burst Cannon and Flamer. That is 3 templats and 9 S5 shots into a Plague Marine Squad. Even though they get their FNP it is nothing they can ignore.

If the Plague Marines charge into them the Crisis Suits have a 3+ save, T4 and 2 Wounds apiece. Yes the Plague Champion with PF will wreck shop the damage will be done. Plus, the turn that the suits come in they still get thier jet pack move in the assault phase so they can Deep strike within 12" of the unit, unleash hell and then fall back getting themselves out of that juicy template target.

This is the only low cost unit other than Drop Pod Sternguard with combimelta that would make me flinch with a Plague Marine Army.

at the place squiggly kills kittens for...or something someone was having trouble defeatnig PM's. this response was the only interesting one:
TH/SS terminators will destroy anything in his army in close combat. They'd be particularly useful against his big, nasty DP.

Against plague marines, you'll need either S8 weapons (preferably AP3 or better) or AP1-2 weapons in droves. Missile launchers, lascannons, plasma guns/cannons, and vindicators all work well here. The trick is to deny him his FNP saves. Another good route is anything with lots of power weapon attacks in close combat, like a command squad or terminators.

Oblits are just 2-wound terminators. A couple lascannon shots, or a squad's worth of small arms fire, should take them down without too much trouble.

It's a tough list, to be sure, but it's far from unbeatable. If he relies on transports (I assume he does), give them a very high priority, and then work on attacking a flank, so that your whole army can focus-fire on 1-2 of his units at a time