I'm another user who have this problem. After I've flashed my N900 with the PR 1.2 update, the Media Player widget was working like a charm, even after reboots the "Play" button would work just fine. But as soon as I restored the apps, the the "Play" button stopped working again.

I've tried removing all widgets from all desktops, only leaving the Media Player widget and some shortcuts. Apparently, it's working again (right after a reboot, no remove / include widget), but I don't get the album art, artist and track name to be properly shown until I press the play button. With other widgets on the desktop(s), the album art, artist and track name would show up just fine.

Gonna try adding widgets to see which one crashes it.