it's definitely all about the room
I have two comparably priced, 'high-end-ish' displays in the same room. a 64" Samsung f8500 plasma, and jvc x35 projector. the plasma is just about as good as it gets, the projector, well it's about as good as it gets at the same 3k price range. due to buying the projector online and not paying taxes, I actually paid about 200 less for the projector.
anyway, with the lights on, no contest the plasma wins. blacks still look black, and it's brighter than I could ever want.
with the lights dimmed, it's a trade off of size for deep blacks. the extra size of the projector is sometimes worth giving up the darkest blacks and shadow details. especially if you're watching something not super dark.
with the lights off, it's projector by a landslide. not only do you get the extra size, but it's actually darker too! i'm sure if I wanted to get picky, I could comment on things like color reproduction, motion, input lag etc, but those are more about the individual models I own, not a plasma vs projector thing.
bottom line is, I feel that the limiting factor is still the content. a 1080p signal is not getting distorted through the projectors lens. the only way you're going to get a more clear image at 100+ inches is with a higher resolution. if they made a 100" plasma it would have some benefits over my projector with regards to brightness, ansi contrast, and bright room use, but I do not believe it could be any more clear.
the truth is, I 'had' to replace my old projector because after getting the new plasma, I knew I was never going to watch anything on the 'crappy' projector again. now, i'm thinking I wasted my money on the plasma cause i'll never watch anything 'critical' on the plasma again. my completely subjective opinion is that the x35 throws out an overall 'better' picture than the f8500.
and that's only 100inches, which is pretty darn small for projectors. my opinion, with today's projectors there is no reason to stick with a plasma for 'theatre' viewing. if you've got the right room, the right lighting control, go with a good projector and screen and don't look back.
Displays: Samsung PN64F8500/JVC X35
AVR: Pioneer VSX-1130K, 7.1/5.1.2 audio
Sources: HTPC, PS3, XBOX360, Wii
Control: Harmony One