I used UMA last month in London and didn't get charged for international roaming - and since I have Tmo H@H service it didn't burn minutes, either.
The only problem I had was that neither my 8320 or my wife's 8120 could get a connection unless we had at least 3 bars of signal strength. Seems the BB gets a much weaker wifi signal than a laptop. I finally just broke down and bought a SIM with cheap rates to the US.
FWIW, I found that I could call the US - using either UMA or my cheap SIM - and connect to my GoogleVoice account and make an international call back to the UK cheaper than calling direct. (Simpler to do that to describe. <G>)