I haven't been to GUM in several years, but as I recall U.S. citizens needed a passport back and forth from HNL. This was both pre- post- (and one memorable trip during) 9/11. As stated above, they have a special visa waiver deal for asian tourists, particularly from Japan and Korea. Trust me, this will all make sense when you get there. Guam caters to Asian tourists more than even HNL.
On the plus side, it's basically one big Duty-Free zone, and, if you're so inclined, it's where Japanese brides-to-be come for their bachelorette parties.