From my HDTV, I was able to pick up free Cinemax HD for a while without the cable box. Since a couple weeks ago, it's gone. I re-scanned and it didn't pick up the channel again. Anybody experienced this?

Comcast does not offer Cinemax HD for free. It may have been in the clear for a brief period, as a mistake. Check in the local topic for your area to see what others are getting.