Perhaps because they saw the thread here about the spread of viruses on USB drives, Microsoft have actually sneaked out a patch in this months' windows updates that changes Autoplay functionality.

In Windows XP, Windows Vista, and Windows Server 2003, AutoRun entries were populated for all devices that had mass storage and had a validly formatted AutoRun.inf file in the root directory. This included CDs, DVDs, USB thumb drives, external hard disks, and any volume that exposed itself as mass storage. This update disables AutoRun entries in AutoPlay, and displays only entries that are populated from CD and DVD drives. Effectively, this prevents AutoPlay from working with USB media.
Just do a Windows update and it should install the fix.

More here:

Update to the AutoPlay functionality in Windows