HAHA @ Rick!
So your gonna open that door huh!
You have no idea how over complex the North America cell phone network is over here! Trust me i had to learn!
They only have Sims on a few of the mobile carriers, T-Mobile, SunCom, Cingular
If its a Verizon, Sprint, Nextel then they store the information physically on the phone. No Sim slot. Thus it wont work in the UK.
You also have to check the spec of the Treo and see if it's TriBand cuz even if it is a GSM one it might be using different frequencies, so you might only end up getting a half decent reception.
But it's a good price!