Regular Season Weekend U.S. National Live Blackout: Due to Major League Baseball exclusivities, live games occurring each Saturday with a scheduled start time after 1:10 PM ET or before 7:05 PM ET and each Sunday with a scheduled start time after 5:00 PM ET, will be blacked out in the United States (including the territories of Guam and the U.S. Virgin Islands). In addition, in the event of extraordinary circumstances that produce a programming conflict, the above blackout windows may be subject to change. If you are an MLB.TV Premium subscriber outside of the United States, each of these games will be available as an archived game as soon as possible after the conclusion of the applicable game. If you are an MLB.TV Premium Subscriber within the United States or an MLB.TV subscriber in any territory, each of these games will be available as an archived game approximately 90 minutes after the conclusion of the applicable game. Archived games are not available through MLB.com At Bat.
It's in the fine print on the sign up page for mlb.tv, it applies ato MLB Extra Innings also.
http://mlb.mlb.com/mlb/subscriptions/index.jsp
ESPN, FOX and TBS negotiated this in 2005 and 2006. It's going to be like this until at least 2013. I live in KY and am a huge Tigers and baseball fan in general and this really screws me on the weekends.