Here's what it is- if you have MLB.tv, you can't watch local games because of blackout restrictions. Here in NYC, the Yanks and Mets each have their own cable tv station that shows all but (usually) one game per week- the other is carried by a local broadcast affiliate (sometimes ESPN, but that's another story). So even with an antenna, you can't watch local games unless they happen to be the one-game-a-week that's broadcast OTA.
YES (the Yankees station) offers a side service that streams these in-market games through the same MLB.tv web interface as out-of-market games. It basically "unlocks" them through the web. Unfortunately, it doesn't do the same thing through any of the "app" versions of MLB.tv (PS3, iPhone, Roku, etc.). Only web, so you don't get full 1080 resolution.
It's kind of berzerk- they offer this service to watch Yankees games on a device other than your TV, but only offer it to people who are already paying to get it on their TV. For an extra cost, no less.