The feds mandated the broadcasting of digital, depending on what market the station is in. The top 10 markets had to be online the first year, followed the top 30 the second year. Not sure the exact rules though, only ABC, CBS, FOX and NBC are broadcasting digital here in Houston - UPN, WB, PBS and the local only channels are not.
The schedule calls for all digital broadcasting by 2006, with the analog lines being turned back in for reassignment. Anybody without a digital set by then would need a STB that can output to an older TV. I suspect that cable boxes will take care of that for most people.