USB port vulnerability. Microsoft's COFEE computer forensics tool is a USB device that plugs into a PC and runs a series of tests and queries against the PC to gather data that can help law enforcement. USB Switchblade is a free alternative available to anyone. Here's a ComputerWorld article on this tool, as well as an article on how to defend against USB attacks (Note: locking your computer does not prevent autorun... you need to log off.)

Easy for someone in your office to walk up to a PC (think cleaning person, but it could be anyone), plug in a USB key, suck off data, passwords, etc, and walk away. My current USB key, for example, is pretty small. Not hard to conceal at all.

Now to figure out how to secure my USB ports...