I lost all my 231-240 channels also, but 240 went the night before, the others last night about the time you said. I rescanned immediately (not from shut down), but did not get them back on Channels 231-240, but in the subchannels 60's and 70's, like 69.14 or 69.15 (not sure if this is perfect as I am at work but something like this). See my thread in the Plasma and LCD section. I really do not know what is going on, but I appreciate your post and know it is not my set. Sounds like Comcast tinkering!!!
Assassin HTPC + Echo, Sony 49XBR850, McIntosh MX151, MC202, MC7104, MS300, B+W 803D2, CWM's, Janus 1W Sub