About a week ago I have installed IE7 build 5335 that was released on March 20. Instead of playing music, Pandora showed a message about that it's taking longer than expected to find next song to play. Firefox solved the "problem". Also the previous build of IE7 was able to play Pandora.
I have mailed Pandora support about the issue and they told me that they are working on IE7 support.
All this means that OpenPandora will not work if you install IE7 build 5335.
For me the solution was to uninstall IE7 and to return to "good" old IE6.