Same case. Initially channels 30,37,38,39,47,48,49 was cut and now most of the channels from 20 to 70 is cut. When asked why is this, I was told the same line "that it was a mistake". I am seriously considering some other kind of service or just cancel comcast service rather than feel cheated.
Now that there is a good deal of online content, I don't see the need for cable myself. I can watch over the air HDTV, or stream HDTV to my plasma, or I could go with basic cable which looks bad on my plasma (since the cheap cable box is SDTV not HDTV) or have to pay extra for the HD cable box.
Anyway, if you are gonna drop comcast and go with someone else see if you can get AT&T U-verse instead. Depending on your area, you may even be able to get fiber at 3.0Mbps for $30 rather than their finicky DSL.