Supposedly, at least 20% of Comcast systems will be moving the entire expanded basic tier to digital by the end of this year, with more to follow. I'm unsure of whether it'll be in the clear or not. I heard something about Comcast contracting for a bunch of cheap converters (it almost sounded like they were supposed to be block converters, but can you actually do that economically with digital?!) but that could just be for the people who don't have QAM tuners. (On the other hand, the several expanded-basic channels that they've just finished moving to digital to make room for more digital carriers aren't in the clear, plus I don't trust corporations to ever do anything in the public interest.)
This also begs the question of what the hell they're going to use all that free bandwidth for. Certainly more HD channels, but what will they be doing til more HD channels actually show up? They'll probably waste it on VOD, plus I think there's something in the works to use several carrier frequencies at once for Internet -- although what exactly they need higher-speed Internet service for isn't exactly clear to me since they seem to frown on customers actually doing anything interesting with it.
For more information, go to broadbandreports.com and find the Comcast TV forum.