Check PalmInfoCenter today, I think there was a TMobile Treo update.

Some of the GSM carriers lock the Treo; Apparently Cingular doesn't. However at the moment if you can find patched.zip (check TreoCentral) you can unlock any Treo, as long as you don't then take another firmware update until the unlock code for your unit is known or an unlock fix for the next version is available.

As far as SIM cards, I think, but do not know, that you should be able to buy a T-Mobile SIM card.