I assume you are talking about the analog channels on Comcast. The regular channels are not encrypted (yet, anyway) so yes any DVR should work fine on them. The scrambled ones would require a Comcast DVR since it has the decoder enabled.
As for the clearQAM digital channels, Comcast in my area (Washington, DC) tends to move those around a lot, so I wind up going thru the channel setup with them about every other month, 30+ minutes each time.