I have a 60" projection TV and I'll share some experience. It's really thick and takes up a lot of space.
I think you can do better then $900 for a 48" TV. Knowing what I know now, I would suggest waiting for the thin DLPs to come down. DLP is truly awesome, there's no screen burn-in and the bulbs are relatively cheap to replace and anyone can do it, plus they have these new ones that are less then a foot thick.
Remember, for HDTV, you also need content. If you have satellite TV, you could be spending a few hundred dollars for a new receiver that can show HD. If you have a Tivo style DVR and you really want to keep that abillity with HD signals, you could end up dishing out over a thousand bucks.
If you just want over the air HD content, expect to spend 300-400 for a receiver for that.
If you think you'll be putting off the purchase of the HD signal stuff until later and that you'll just use the HDTV as a normal TV for now, it might be worth waiting on the purchase. The prices are dropping quickly and waiting another month or two could be the difference between getting it now versus a generation of technology and a significant savings.
I look forward to selling my 60" projection sometime to replace with DLP. It's just too cool.
PS, plasma sucks. Look cool and I wouldn't turn down a free one, but a TV that has to be replaced every few years? Gracias, para no gracias.