The primary digital signal being broadcast by most stations is a simulcast to the analog, so for the most part, getting a digital converter wouldn't give you anything you didn't already get. Some stations are "multicasting" -- i.e, in addition to (or in place of) the HD signal, they are providing additional streams of programming -- a weather channel or possibly another network's feed (in some places, for example, a CBS station is using its multicast capacity to transmit CW programming).
According the NTIA (National Telecommunications and Information Administration, an office within the Dept of Commerce that is administering the converter box coupon program), nineteen boxes are currently being tested as part of the certification process. These boxes should start appearing in stores next month, which is also when the coupons start being sent out.
http://www.ntia.doc.gov/ntiahome/press/2008/DTVretailers_010708.htmlI don't want anyone to think that I believe this process is being handled well or is going to run smoothly. Anything but. People are going to be confused, and there are going to be problems. One problem that already is rearing its head is that Congrss, in its infinite wisdom, decided that the coupons should expire within 90 days of the date they are mailed, instead of allowing them to remain valid until some set date following the end of the transition (such as 6 mos after Feb 17, 2009). Big mistake and one that Congress might yet still try to fix, from what I've heard.
The biggest problem however is that, as others have mentioned, the folks most likely to rely on over the air reception rather than cable and satellite, and thus the folks who will be most directly impacted, are also the folks that are least likely to hear about and/or clearly understand what's going to happen: the elderly, the poor, the non-English speaking. Outreach to these groups is underway through AARP, various other organizations that communicate with immigrant communities, etc, but in the end, people will fall through the cracks.