Will a UK Android telephone work in Japan? I am on the UK carrier Vodafone, and wish to upgrade my phone soon. I am going to Japan for five months next year, and would ideally like a phone which I can use out there, with a Japanese sim card.

Will an android phone I get here (e.g. the HTC Desire) work in Japan? Will it need to be unlocked?
Sim-locked phones need to be unlocked for use on other carriers.  Almost every phone you get will be sim-locked.  The most easily unlocked phones I know of are the Galaxy S line.

The only other thing you need to check is the radio frequency bands used by the device and the carrier.  If they match, you're in luck.  "Quad-band" devices should work on every GSM network.