RF IDeas, Inc. offers proximity activated indentification devices.

The pcProx-Sonar device automates the locking of the PC when the user step away. When approaching the PC the pcProx sends keystrokes to the OS and automatically logs the user in. Ultra-Cool! pcProx attaches to the PC via the USB port and is configured by the system as a keyboard.

For example, under Windows 2000 the pcProx sends the sequence (Ctrl-Alt-Del K) when the user leaves the PC. As the user approaches the PC, another keystroke sequence is sent to ready the PC for password entry.

The pcProx sonar measures 1.75x1.75 x 7/8 inches and sells for $129.00. Coupled with an ID card reader the user can be identified automatically without logging in.
More details on RF ideas product page.