When they type w910, they mean w910i.
And btw, that phone is not only not available with a carrier here, it's not even available... period. But that's okay, I bough a european k750i some years ago and it worked fine.
I is compatible with US bands, but only at EDGE (2.5G) speeds. The w910i does have 3G and 3.5G, but for your phone, it only works in Europe.
I just purchased a k850i last month and it has ALL the US and European bands, if that's what you need.

w910i:
* GSM 850 US
* EDGE
* HSDPA 2100
* UMTS 2100
* GSM 900
* GSM 1800
* GSM 1900 US
k850i:
* EDGE
* GSM 850 US
* UMTS 2100
* UMTS 1900 US
* HSDPA (they didn't specify what band this is, but I can tell you from experience it works in the US, it's probably 1900)
* UMTS 850 US
* GSM 900
* GSM 1800
* GSM 1900 US

look on sony ericsson's website for more options, in my opinion, they are the best phones in the world