You could go either way to be honest, anything bigger than 42 inches I'd say go plasma, smaller I'd say LCD, but at 42 in you could go either way. I think it would come down to weighing factors between the individuals at this point because both plasmas and LCDs have their cons for gaming.
LCDs of larger size typically produce a softer, less sharp image when running in 480i/p in particular, they are slightly more expensive at this size as well, and they often have narrower veiwing angles (not so much an issue for gaming though).
Plasmas do have a potentional for burn-in (though not as bad as it was), they generally have fewer connection options, and they typically have lower native resolutions than LCDs (that has been improved as well somehwat but a 1080p plasma will cost you an arm and a leg more than a 1080p LCD or projection at the moment).
So in the end I'd say look at the options and features of the individual sets because both technologies have their little caveats. If something has much better connection options, or better speakers (if you around using a SS setup), or does video processing cause lag, or does it has better black levels, or color control and contrast, or better menu options for easy adjeustments, etc. I'd be looking for things of those nature. A suggestion maybe to take you 360 with you and hook it into the TVs you are looking at, this way you get the best idea of what the picture will look like for your gaming before you take it home. The vast majority of TV sales people will oblige you particularly if it is a larger more expensive set, if the hesitate tell them your thinking about buying service (you dont have too in the end) and that generally lets you get away with more.