HD Baseball on Comcast
Not sure if this is a regional answer, but when I had Comast HD last year, they broadcast all Atlanta BRaves home games on a channel labeled BRAVESHD - can't remember the channel number, but then that is probably different by market. It was only "live" when the Braves were playing (or scheduled to play). If there was a rain delay, the cameras and audio would stay live on the high-home-plate camera and you could occassionaly hear the announcers joking around or discussing what they would say when they came out of commercial. As soon as the game was over and at all times that there was no scheduled game, the screen would just show a BRAVESHD banner.
The HD feed was independent of which network was nationally broadcasting the game. It was always just changed to display the proper logos and graphics.
I hope this answers your question...