All broadcast TV is not going HDTV. It's going Digital.
Currently over the air signals are analog. That will change in February, when these signals go Digital.
So only the oldest, non digital TVs, not hooked up to a cable or satellite box will not work. I can't beleive there are that many out there.
I haven't given it much thought, but I think the TV companies might be behind this whole thing. People can get $80 towards the purchase of either a converter box or a new TV. What do you think most people will do? Yup, buy a new TV.
Here's whats happening
http://www.fcc.gov/cgb/consumerfacts/digitaltv.html