Disclaimer: here follows one of my regular "DON'T BUY NOW! WAIT AND SEE!" posts. Everyone else seems to have said the same. It's also true that if you're always waiting for whats's around the corner, you'll never buy anything.
That said though, the time to buy a display for an XBOX 360 is
when you have an XBOX 360 to play it on. I reckon loads of people bought FX5800s for Doom 3 / 9800XTs for HL2 on the basis that "they're designed for them!!!1!11!" only to find that by the time the games actually arrived the next gen cards were about to arrive and were loads better.
The important thing is that it's very likely that most/all HD video sources available to us in the UK will require an HDCP compliant display- and AFAIK
none of the displays currently on sale can guarantee that they'll be able to comply. I'm not 100% up to date with what video outputs the 360 will have when it ships- but I have some idea that it won't even have
any digital video output when it arrives. I really don't know if standard analogue component outputs can cope with resolutions higher than standard 576p (does anyone?- please LMK if you do

).
I intend to be (if I can afford it) one of the HDTV early adopters, because from what I've seen it looks stunningly good

. Right now though, I have a very nice 29" Sony 4:3 screen I bought in 1999 which still does me fine. If I was really hurting for a new screen I'd lay out £400 for a 32" CRT in the knowledge that it's fine for now, and over the next year or so it can't really depreciate by more than £150-200 in resale value. Someone who buys a £1200 plasma or a £2000 DLP rear projector is taking the risk that it will end up incompatible with the new standards and they stand to lose serious money (like £500-1000) if they need to sell up and buy something new in a year's time.
As a final wildcard, discussions have recently started about the standard that will replace DVI. As HDMI is DVI with added sound and Content Protection that might go soon too.
Wait and see. Wait and see

.