I work in a computer store as a Technician and get this question asked a lot.
All depends on what you're doing. If you just browse the web and check email, theres absolutely NO point in getting a giant hd tv to use as a computer monitor replacement. Now, if you use your computer for a lot of different things such gaming, movie-watching, video editing, photo-editing, or hell, even one of these things, then yeah, you may want to get a nice hd tv. It doesn't have to be a big giant 42" screen, they do make very nice 19" hd monitors.
Keep in mind, you also will need the hardware to take advantage of your nice new shiny screen. Your old compaq presario thats been sitting in your basement collecting dust certainly isn't going to put out 1080p video.