They probably won't be doing that in anything close to a short while, and the reason is that they still have a significant number of customers without converters.
Uh... Comcast is already doing it--see my post above. Anything above analog basic (Chs. 2-31) will be snow unless you have a box (and, of course, the subscription). Vancouver Comcast customers had to get their boxes by March 18, the rest of the Portland area is to follow this spring. Comcast will be providing two digital boxes to every customer who doesn't already have one.
