If you live in the city of Chicago and have Comcast Cable, the MLB Network showed up on digital channel 410 when I flipped to that channel this afternoon.
At this moment (1:40 pm CST Saturday) they are showing something about the Red Sox.
Don't know what they'll be showing between now and the official launch on January 1, but it's a good place to get a baseball fix on these cold, snowy days. Not sure if there's a HD version yet. Here's the MLB Network official site.