When trying to debug a managed package I'm seeing the following message in the Debug Logs.
Log filter in effect. Executing code from installed managed application
This is "by design" and means you can't see debug level log messages - http://community.salesforce.com/sforce/board/message?board.id=apex&message.id=12460#M12460
Update 13/02/2012
The idea Display managed package logs in subscriber orgs is marked as delivered but doesn't actually say what was delivered. I've raised the following question on the forum to try and find out - Logging from a deployed managed package.