I was happily watching MLB.tv using my Mandriva laptop with Firefox 1.5 and mplayer plugin, but as soon as opening day hit, MLB launched their new interface, which I believe requires Flash 8+, which is not available for Linux, thus this killed my streaming baseball experience. Needless to say, I was pissed, so I spent endless hours tweaking and trolling the Linux boards to find the solution.
Below is the simple solution that got it working for me:
1. Add noembed=1 to $HOME/.mplayer/mplayerplug-in.conf
2. Using native firefox (linux version), go to mlb.com and at the top of the page log in. The key is to log in first.
3. Then select a game and mplayer will launch a window.
This will allow mplayer to open the streaming video in a separate window. Below are links to the message threads you can reference for more details.