Playstation Vita released today - Sony failed to publish any network support specs (website, downloadable doco, media releases, Sony Shop staff) - I tried my XT SIM in one - works great. Not sure if it would have been on 850MHz or 2100MHz though. Assuming it is the same specs as the AT&T model released in the USA. Should work on both :)

BTW - Price difference between Wifi and Wifi+3G model in USA is USD$50 (NZD$60).  Here we pay NZD$150 for this feature.  :(