I heard the same thing Jack mentioned about the 85%.
Our local CBS station has some Q&A on their website concerning HDTV. This is what they had to say...
Why are we changing to HDTV now?
Congress passed a law mandating all television stations broadcast a digital signal by the year 2002. KFDM Channel 6 is currently broadcasting both the new high definition digital signal and the regular analog signal. We, along with other broadcasters will simultcast digital and analog signals until 85% of our viewing households have purchased a digital television. Then, our analog signal will be turned off and our broadcast frequencies will be turned over to the government. Once the digital conversion has been completed, Congress will use the old broadcast band widths to expand usage of cellular phones, data transmission and other electronics applications.
If you would like to read any of the other Q&A they have, you can see it here...
http://www.kfdm1.com/six_HDTV.shtml
Most of what they say is pretty genenral, but you might find something of use in there.