It's supposed to be August 2011, but at that point it really depends.

For cable (I include satelite here) not much will change, the cable company will likely take in a digital signal, convert to analog, then send that down the existing wire to analog sets.  At least at first, since it takes extra equipment to do the conversion, expect the cable companies to go "digital only" after a while reversing the trend, traditional basic cable stations will be sent in Clear QAM, and you will need a set top box to work with analog receivers.  Then it becomes similar to OTA.

For Over-The-Air, it's going to mean either a set top digital to analog conversion box, or a new TV.  Set top boxes, where available run about $100, which becomes the deciding factor.  Something like a 3 year old 60" Plasma that cost $2,000 when new and is still working perfectly might be a candidate for a set top box.  A 9 year old 20" bedroom TV might be replaced, and the old B&W you inherited from grandma, and you leave in the spare room for when relatives visit, may simply be retired. 

What will be interesting is the VCR, some set top boxes can be set to come on at a certain time, and the VCR is then set to come on and record, some set top boxes can not be  set that way, so I expect a lot of VCR's will be retired/replaced as well.  I have one that is a candidate for such, and the game plan is to replace it in 2010 with a home built PVR.