I have cell/data service (Treo) through Cingular and an aircard through Verizon. They (aircards) are great for places where you're unsure of the WiFi situation. Also, I'd rather pay $80/month and have service everywhere than have to hit-and-miss at $40/month on T-Mobile or pay $10/day to random providers.

I imagine that the Cingular cards work about the same as Verizon cards do (besides the fact that Cingular is GSM/Edge and Verizon is CDMA/EVDO). But I think the rub you'll run into is the data roaming rates in Europe. You may buy an unlimited data package here in the US...but you'll pay through the nose for data roaming in Europe. You'd probably be better off looking into data plans/coverage from European cell companies rather than Cingular. Try Googling for Vodaphone and Orange.