Here's a post on Flash banner ads that hijack your clipboard, and won't let go until you've restarted your browser (or your OS, depending on who's talking...): http://blogs.zdnet.com/security/?p=1733

And here's the thread on Apple's support boards - seems the first user to publicize this was on a Mac:
http://discussions.apple.com/thread.jspa?messageID=7768848