Reports are coming in that people visiting http://discussions.apple.com have been seeing the cryptic message "for fun, by tojen" rather than the index of discussion forums they were expecting.
We've managed to reproduce this here at TUAW, but only haphazardly. I saw it earlier in Safari under OS X, but since clearing my cache I cannot reproduce it. Firefox doesn't do it, but lynx (a command line browser) on my Linux server does. Steve Sande has seen it under Safari but not on his iPad. Other people have reported seeing it on their iPhones.
This mixture of affected devices and operating systems that show the message suggest it is neither an OS X nor a Safari hack of any kind. If you have seen it, your computer has not been compromised. Rather, it's something server-side; either on Apple's servers themselves, within the Akamai caching network (this would be my guess based on some quick tests), or some sort of DNS hijack.
For the time being I'd suggest being cautious and steering clear of discussions.apple.com. However, don't panic. If this was a hack aimed at stealing your accounts then it wouldn't put up the message at all, it would show you the normal page and sit quietly in the background accumulating data.
Nevertheless if you've seen this message and are worrying about the implications, you should consider changing your iTunes account password, if only for peace of mind. It is just about possible that your session cookie will have been intercepted by "tojen" for nefarious purposes and changing your password will protect you against that.
Thanks to everyone who sent this in.
Update: the whole discussion forum is now closed, replaced with the cheerful "we'll be back soon" sticky note normally reserved for the Apple Store. Presumably a number of system administrators just had their Saturday evening plans cancelled. (Thanks to commenters snoanim and John for pointing this out.)
Update: Forums are back.