The Pentagon has approved just one single Android device though. The Dell Venue is the only device to make the cut, and even then, it must be running Android 2.2 and have the Android Market removed before it can be used by DoD staff. Additionally, any web browsing done on the device must be routed through the DoD proxy server.
The Pentagon is still testing iOS devices, but has not approved any because it is more difficult to load custom security software into Apple's walled garden. Government officials also cited Apple's location tracking as a sticking point in approving iOS for use.