As a person who has been holding off on buying a HDTV for myself (although I picked out a TV for my parents, aunt/uncle, and 3 friends) I have a little experience in this. First of all avsforums is the place to go, but it will be information overload. You need to ask yourself a few basic questions:
1) Does it need to be wall hangible? (if "yes" you have just narrowed yourself from CRT's, DLP's, LCoS, front projectors, LCD's, and Plasmas to just Plasmas and LCD's).
2) Are their any windows in the room that the TV will be located?
If yes, do you have blinds or curtains that will allow you to control the light? (If no, you just about removed LCD's from being a choice if you want to use it during the day)
Do these windows allow the sunlight to glance off the TV? (if yes, they you just about removed LCD's from being a good choice if you want to use it during the day)
3) How wide of a viewing angle do you need? (i.e. do you basically sit directly in front of the TV or is there a lot of seating that is comming from the sides?) If you have a lot of side seating, you remove most of the cheap LCD's from being good TV's (cheap is a relative term here, highend LCD's in the 40" range are still in the $4000-6000 range, so a lot of the $2000 ones skimp on their panel and lose off axis viewing, same is true on some of the plasmas to an extent, but in the plasma case you wind up with a double image as if you looked at the reflection in a double-paned glass window from the side)
4) Do you want to connect a computer to it for use as a display? (this one is HUGE... if you answer yes to it, you just removed all plasmas from the list and most LCD's as well. Only a few LCD's have a DVI-I or VGA port (yes, you can get a convertion cable for HDMI, but there have been problems with HDCP) and even fewer of the already few 1080p displays actually allow a 1920x1080 resolution to be sent to the screen from the computer and instead limit you to 1024x768.... in fact only 3 models exist to my knowledge)
5) Are you planning on upgrading to Blu-Ray DVD or HD-DVD in the near future (getting a PS3 will be considered a yes)? If yes, should go for a 1080p display. But make sure that it is one that can accept a 1080p signal from HDMI or DVI input, NOT ALL CAN, even though they have a 1080p panel, you can only feed it with a 1080i or 720p signal which means that you might as well have just gotten a 1080i panel or 720p panel since it is COMPLETELY USELESS)