Fortunately, I got a big read-through on this a while ago.
First of all, if it's LCD-based, you're just about in the clear. Just don't leave it on for a few days or play one 16-bit game straight through without pausing.
If it's CRT-based, you start running some real risks.
Here's the thing : Regardless, you TV WILL burn in. Just being on burns in. Now, the thing is, for the most part, this happens to the entire screen, over a large variety of colors. Things like wide-screen-style (or full-screen style if it's a widescreen TV) black bars will leave a burn-in-like effect, due to the fact that everything but the bars burns in over time.
Now, these things take a LONG, LONG time. You could probably play through Zelda : Wind Waker twice before you noticed something. Of course, once it's there, you're screwed.
If you want to take the safe route, just turn the brightness 'n contrast down before playing a game. That really lessens the effect. Also, of course, mix some TV time into your game time from day to day, or best-of-all, stick to games that don't have any bright, static status images (health, magic, bullets, etc.) on there that don't slide or fade out when they're not in use.
Then again, you're getting a 46" projection screne for sub $100?! :: jealous pout:: Screw you man, if that thing *exploded* within a year you'd still be better than most of us~! =P