Metagaming can be a problem.
In the campaign that I'm DMing, I don't allow players to look up stats of monsters, or leaf through random books trying to find a way out of the current situation. They're pretty good about playing under my rules, and metagaming doesn't happen very often as a result. I ALSO do tricks like Doc has mentioned, and I change stats for monsters all the time. (However, while I might change the DR for a specific creature, I wouldn't change the fundamental rules behind it without at least mentioning it to the players.) I never tell them specifically what they're facing unless they've encountered them before. Even then, I might say "You see a rat-man creature similar to the two you killed a few minutes ago."
HOWEVER, I've seen how much a problem it can be.
In a campaign where I'm a PLAYER... last night we ran into a Nightshade. We'd never seen ANYTHING like this before. After a couple of rounds not hurting it, the DM mentions to everyone that it has 25/+3 DR.
IMMEDIATELY, all the players bemoaned that they couldn't hurt the thing (we didn't have any +3 weapons) and started trying to figure out ways to do more than 25 points of damage. The party had ONE +5 weapon in the form of an artifact mace, and immediately everyone tried buffing up that character as much as possible.
In the GAME - our characters had maybe taken two swipes at the thing, and a pittifully failed Knowledge: Undead check would not have revealed its DR to us.
Well, I'm not one for metagaming. My character spent 3 rounds swinging at it, and missing - but he didn't know his weapons would be ineffective, so he kept attacking. Then he finally hit twice, and rolled near maximum damage each time. Thus, he realized that his weapons weren't strong enough to hurt the beast. When the Nightshade used its Finger of Death ability on the character with the artifact, I jumped down, grabbed it, and switched to that weapon. We barely managed to kill it before it fled.
My point? Well, our DM in that group really doesn't care one way or another if we metagame... I do though.
EDIT: Goddamnit I actually used the word "Buff" when describing enhancement spells. ARGH! See what bad influences you guys are on me??
