From Feb 17th, 2009 USA is going digital. The full-power TV stations will stop broadcasting in analog and switch to 100% digital broadcasting. The useful resource would be: http://www.DTV.gov. If your TVs are connected to cable, they will continue to display [the same/full-power/broadcast] stations. However, analog-only TVs that receive those stations via an antenna won't display them after 2/17/09 unless you take action. (Those TVs should continue to display low-power, Class-A and translator TV stations and work with gaming consoles, VCRs, DVD players, etc.)
Either you have to connect them for Cable, or replace those TVs or buy converter boxes that will allow them to display DTV signals. The NTIA will provide up to two $40 coupons per household for the purchase of such boxes. For information, visit https://www.dtv2009.gov or call 888-DTV-2009.
I found this one nice and informative: