Last night, on Patch Tuesday, Microsoft distributed a security fix that disables Autorun with USB drives and other forms of removable storage on Windows Vista, XP and 2000, and Server 2008 and 2003. Windows 7 by default has Autorun disabled for removable storage, so this is just bringing the older OSes into line.
This marks the end of an era, at least as far as worms like Conficker are concerned. Historically, all a virus had to do was place a a very basic Autorun.inf file in the root of a removable drive, and it would be automatically executed upon its insertion. As you can imagine, this was a very, very easy attack vector, and incredibly effective too.
Incidentally, because Microsoft says it hasn't seen an in-the-wild malware attack that uses CDs or DVDs, AutoPlay will still work with "shiny media."