The U.S. Constitution requires a voter to be a resident in one of the 50 States or in the District of Columbia to vote in Federal elections.
The exceptions are if they are serving somewhere in Puerto Rico, Guam, American Samoa, and the U.S. Virgin Islands. (Under UOCAVA)
Edit - If voting isn't a right, then why so much contoversy over this subject? We are not denying them their rights, so whats the issue?