I seem to remember that early-production plasmas had some trouble with burn in. I'm not sure, but I think that has subsequently been reduced/fixed. LCDs do not and I don't believe have ever had a burn in problem.

I'm not sure if this is still a factor, but I believe at the 40-42 inch size that you're talking about, plasma will be a better price. I might be going on older knowledge, but I think LCD prices tend to increase a great deal at a certain size.
I don't know if that's even a factor for you.

I can tell you, for sure, is that you need to keep your eyes open for good deals. If you watch, you can score big time.
I upgraded to a 34 inch HD LCD a few months ago (from a 19 inch CRT). I paid $1000 at Best Buy (they were advertising a $100 price drop) and the next week, it was in the Best Buy ads again with another $100 price drop.