I remember a dude who used it all over Europe by putting a prepaid SIM from Europe into his BB. He said as long as he was on wifi he was good on UMA and the minutes just came off of his plan. What I don't remember is whether he was Rogers or Tmob, but it was posted on this forum maybe 4 months ago. I'm pretty sure it was the 8900 forum because that's the only one I visit often, but I have used the Bold and 8100 forums too at times.
BB 8900 Tmobile, Moto Droid Verizon