The SANS Institute recently published a document discussing Universal Serial Bus (USB) drives (also called Thumb Drives, Flash Drives, or USB Keys) and their implications on enterprise security. It's a good overview of the basic threats, types of attacks, as well as common risk mitigation strategies.
It's titled: USB - Ubiquitous Security Backdoor
Despite the lame title, if you're trying to make sense of the threat posed by flash memory drives, it's worth a look. Of course, if you're already a security guru, you can give this one a pass.