There’s a lot of incorrect information floating around this thread. I’m not a DTV expert, but I’ve done enough research to try to straighten out a few errors of fact.
Because the HDTV format is configured that way. The main reason for that is to make it better suited to Hollywood movies, which are roughly the same aspect ratio.
This is incorrect. As Markxxx says, Digital TV is not the same as HDTV. The FCC mandated the conversion to digital TV (I think by 2006, although that may have been changed) but broadcasters are not being forced to make all channels HDTV. Some (probably many) will be Standard Definition Digital TV (SDTV), basically a digital version of the current TV picture–a 4:3 image! (See the link on Markxxx’s post for all the gory details.) So come the digital age, we will have new 4:3 programming.
No, there are 4:3 TVs that are capable of showing HD programming in widescreen format with black bars top and bottom.
As Max Torque has pointed out, a widescreen TV will have black bars on the sides when it displays 4:3 pictures. The black bars (along one dimension or the other) are the inevtiable result of trying to fit a square picture in a rectangular frame, or vice versa, without distortion or loss of information.
A final important factor that hasn’t been discussed here is receiving broadcast HDTV signals. As with most things digital, it’s all or nothing. If you expect to receive over-the-air broadcasts (with an antenna, as opposed to via your cable TV system), you should carefully research signal strengths at your location. If you’re too far from the transmitter you will not get a weak or fuzzy picture, like with analog TV. You will get nothing. Overcoming this problem is tricky. It may come down to making sure the store has a good return policy.
Fortunately, more and more cable systems are offering more and more HDTV channels so reliance on broadcast signals is becoming increasingly rare.
It’s a confusing situation, no doubt, and that’s one of the reasons things haven’t progressed more quickly. But with a little reading (okay, a lot of reading!), like Markxxx’s link, you can make an informed decision.