Jacqui relates the amusing story of someone purchased a refreshed mac, only to find the desktop covered with porn links. She's skeptical, and I can appreciate her caution in the face of a story that reeks of urban legend.
In my case, I didn't have a desktop full of porn, but something (presumably my disk) had not been cleaned. The test drive version of Office, for example, is convinced I'm Swedish. The shell is convinced my machine is called "bench3-3", a name I never typed in.
The big problem with this isn't that you might find annoying content on your desktop (though some such content is illegal, and even if you delete it, it might show up in a forensic search), but that returning a failed computer to apple might expose confidential information to whoever eventually gets the disk. This is particularly disturbing because when a customer returns a broken system, they may not have the opportunity to clear the disks themselves.