I am currently stationed in Japan and, suprisingly, there is not a huge selection of PDA phones available here. I have seen a couple on the AT&T website that I'd love to get, but concerned about buying one and not being of use over here. Question is, can I purchase an "unlocked" phone and use a sim chip from a Japanese company (DOCOMO, SOFTBANK, AU, ect) while here, then change it to AT&T service when I get back to the US?
Japan is not GSM, but it does have 2100mhz UMTS. I have used a Nokia E61 and and an HTC TyTn a/k/a Cingular 8525 a/k/a Softbank x01HT there without a problem. Softbank and DoCommo have 3g nets which are fully compatible with this phone. Buying a phone off base has proven problematic for some servicemen. Japan requires proof of residency to get phones and while your orders should work, they often don't. If you can buy the SIM card on base, great. If not, I'd ask around to find a recommendation of a store that is friendly to serviceman.
Here is a link to Softbank's English language site:
Here is Docomo's:
Softbank has announced eight new mobiles coming out over the next few months. You might want to look at these.
Rather than buying an ATT phone without a contract, look at the various US vendors of unlocked phones (or take leave in Hong Kong once). Here is a link to one vendor of unlocked phones but there are many more:
Depending on the base, there may be a cell provider on base that will give you a plan as long as you have a local mailing address (and maybe your orders). At Yokota, there is a Softbank booth in the BX building.
If you want to get an AT&T phone (unlocked or unlock it yourself) your choices are the Tilt, Blackjack 2, and Treo 750. Other options may be available from online vendors, etc., but make sure they work on the UMTS 2100 mhz network (the large majority of US phones do not).
The cheapest choice would probably be to upgrade through AT&T (if you are eligible) and request the unlock code. From what I have read, they are pretty reasonable about this if you tell them you are going overseas (probably especially so if you are military). Although I think this takes a few weeks? Alternately, there are unlock services on the Internet.
As others have said, you can buy an unlocked phone from alternate sources, but you will be paying full price.
As mentioned, you can also get a Japanese phone. Make sure it has the required GSM frequency (as mentioned above) if you intend on using it stateside. It would need to be unlock also, I believe.