Most U.S. TV stations already broadcast digital signals as well as analog. What's happening a year from Sunday is they'll switch off the analog signals. No one with cable or satellite service will be affected, nor will anyone who gets stations over the air with a newer TV with a digital tuner.
(image credit: AP/Matt Rourke)
PS: In the UK, analog TV will cease on different dates for different regions, with the last signals ceasing in 2012. -Thanks, violet/riga!