Technically, nothing's stopping you. But in theory, carrier policy might get in the way. UMA technology makes it possible for the carrier's to decide what should happen.
You see, when an UMA phone starts up, it is required to scan the cellular network first, before it does anything else. Then, it will try to get UMA service. This will happen even if you've configured the device to only use WiFi.
As part of the registration process towards the UNC (this is the carrier's server ,the one that acts like a cell tower, only over the Internet), the mobile will report the identity of the surrounding network. Part of this report is the MCC, or mobile country code of the network. Using this information, the carrier can easily figure out what country you are in. If they have a database of the exact cells in the area, they could figure out where you are to within a 30 km radius too.
If that doesn't work, devices equipped with GPS will generally report your coordinates as well. This is all happening as soon as you power up the phone.
So if there is cellular coverage, your home carrier will be able to figure out where in the world you are. They can then comply with existing roaming agreements with the carrier in the country you are in, or they could just be evil and charge you more, keeping all the money for themselves, since they don't even need the other carrier.
I suppose you could make sure the phone is out of cellular coverage somehow. Maybe you could wrap your hotel room in aluminum foil. But then the mobile will report that too. It will say that it's out of coverage, and your carrier will know that you are not at home from your IP address, and they may refuse you service.
Here's an ideal scheme: Suppose you have a lot of family in the US, but you live in the UK. So you go to the US, sign up for UMA service from T-Mobile, and they give you a handset and an access point. Say thanks, and then go back home. Plug that AP into your existing broadband internet, and you can now make calls to the US at local rates. This assumes that your Internet bandwidth is cheap, however.
So right now, you can beat the system. But when travelling, I'd take an extra roll of aluminum foil, just in case...