Recently I've experienced the same problems as you all with the media player widget "play" button not functioning, and after trying a few things, I think I MAY have found a solution. Please confirm if possible.

Rather than deleting the widget and replacing it on there, I found that (maybe just after PR 1.2?) deleting my Conversations Inbox widget from my desktop made my media player widget work. Now it's currently still installed on my N900 (Conversations widget), but the widget is no longer placed anywhere on my desktops. As of then, I've had no problems with the "play" button of my media player widget, although I'm afraid to go as far as to saying it could work for the rest of you.

This is simply something that I've been able to confirm, for myself, twice - once after removing it and feeling like the Conversations widget made life simpler, only to find out my media player stopped functioning again.

Please try this out for yourselves, after all, it only involves removing that one widget away. At the most, possibly an additional reboot to take care of the problem once and for all, (although I don't quite remember having to do so). Let me know how it goes!

edit: If this still doesn't work, the only widgets that I have on my desktop are calendar, e-mail, Facebook, and media player.