RTFA: http://blogs.zdnet.com/security/?p=1733

Malicious hackers are using booby-trapped Flash banner ads to hijack clipboards for use in rogue security software attacks.

In the Web attacks, which target Mac, Windows and Linux users running Firefox, IE and Safari, hackers are seizing control of the machine’s clipboard and using a hard-to-delete URL that points to a fake anti-virus program.

According to victims on several Web forums, the attack is coming from Adobe Flash-based advertising on legitimate sites – including Newsweek, Digg and MSNBC.com.

Here is a Mac OS X user explaining the attack:

This has happened to me twice now, on two separate computers at work. My clipboard has been hijacked with this:

[ malicious URL deleted ]

And once it’s in the clipboard, I can’t copy anything else over it until I’ve restarted the machine.

…according to the slashdot threads, this isn’t quite as cross-platform as they’d want you to believe.