It kind of irks me how people who game a lot are more worried about burn-in than input lag. IR can happen, but it's temporary and in practice isn't even noticable except on single-color screens or coming off a long period of black bars. As has been said, CRTs carry the same risk of burn-in, and unless you're gonna use your display as an airport or arcade monitor you won't get it.
A friend of mine has an LG, notorious for bad IR, and we've done numerous marathon sessions of Resident Eveil 5 (for example) without any lasting problems. I'll deal with some temporary IR if Mario jumps and Guile throws a sonic boom right when I push the button :P
So in short, if a plasma fits your budget and you find gaming on the one you choose to be quick and responsive, go for it.