Contrary to what you might imagine, the biggest cause of TV-induced eyestrain is watching a small, overly bright television in a dark room. If you've ever had a headache from watching Letterman while on the way to dreamland, you can probably sympathize. Changing any of these factors--by getting a bigger TV or sitting closer to the screen, by reducing the TV's light output (via the contrast control), or by increasing the ambient light, preferably with a dim bulb placed behind the TV--can help reduce eyestrain. The type of TV is a factor as well, but only so far.
Older CRTs have a slower scan rate of 15KHz, which refers to the speed at which the image is drawn on the screen. In these sets, you can actually hear the high-pitched whine of the picture being drawn. Just as with computer monitors, a slower scan rate is harder on the eyes since it actually flickers, even through the flicker is too fast for most people to detect.
All TVs capable of displaying HDTV and progressive-scan DVD, including newer CRTs, plasmas, LCDs, and most big-screen rear-projection sets, either have higher scan rates that don't flicker visibly, such as HDTV-compatible CRTs, or use a completely different method of display that is onscreen constantly instead of being "drawn," as is the case with the others. In this way, they're all much easier on the eyes than older analog CRTs.
In my experience, plasma is significantly easier on the eyes than CRT, since it's generally bigger and has a higher refresh rate. That said, if you sit far enough away from a bright-enough plasma in a dark-enough room, you'll still experience eyestrain.