As of May 2003, more than 1,000 stations were on the air with DTV signals, and every major TV market was served by at least one DTV station. The target date set by Congress for the completion of the transition to DTV is December 31, 2006. However, that date may be extended until most homes (85%) in an area are able to watch the DTV programming. At that point, broadcasting on the analog channels will end and that spectrum will be put to other uses. Until the transition to DTV is completed, television stations are required to broadcast on both their digital and analog channels.
Timeline:
The FCC (Federal Communications Commission) has mandated that all TV broadcasts must be in Digital format by the year 2006. What does the timeline look like from now until then?
May, 2001: Network affiliates in the thirty largest markets must begin digital broadcasts.
May 2002: All commercial stations in the US must begin digital broadcasts.
May 2003: All noncommercial stations in the US must begin digital broadcasts.
May 2006: Analog TV signals will be completely eliminated in the US.
From the present to May, 2006 Broadcasters who begin DTV transmissions, will simultaneously broadcast analog TV as well.
Also take note that these mandates have not stated anything about satellite or cable broadcasts nor high definition broadcasts. It is up to the cable companies to make the switch to digital on their own. As for the actual TV stations, they are only required to broadcast in a digital format and broadcasting in high definition is completely up to them.
Another interesting thing to note is how long it took most stations to get up to snuff. There still are a lot of stations that are not broadcasting digitally, though most local affilates are on the way.
A good read:
http://www.hdtvpub.com/