I have an ATT (Sierra) USB 881. I download Sierra's software and it had configurations for most carriers. I've used it with ATT, 3 UK, Etisilat, Vodafone, and 3 HK.

You should generally be ok. The thing you should be aware of is that there are five frequencies (800/900/1700/1900/2100mhz) of HSPA, but most modems only support three out of five. Most European HSPA modems that are global have 900/1900/2100mhz. TMobile USA is the only US carrier that uses 1700mhz and you can go without it. You really want 800(sometimes called 850)mhz.

My personal suggestion is to buy an unlocked US unit that has 2100mhz. I would rather try to go without 900mhz HSPA than 800mhz. 900mhz is pretty populated with 2g carriers, and I think most 3g action is on 2100mhz.

Do you research on this one, I'm speaking generally. Your situation could be different.