The service, available via a free update, requires an MLB.TV Premium subscription and will cost $35 for the remainder of the 2009 season.
One drawback (kinda, sorta, but not really) is that your only allowed to stream out-of-market games, so you won’t be able to watch your local team play on the Roku.
MLB games can also be streamed on the iPhone with the MLB At Bat 2009 application.