According to this web site it's the Secret Service chronograph.
"The Secret Service chronograph given to Senator Obama by the Secret Service agents is a private label watch available only in the Secret Service employees' store.
The watch is manufactured for the Secret Service store by Jorg Gray, and appears in the Jorg Gray online catalog as the JGC6500 Chronograph.
The chronograph is made in China, with a Japanese movement. (Click on any of the following images, to see an enlargement.)"
Barack Obama's TAG-Heuer