It's a funny old world
In the USA T-MO offer seamless transition from GSM/Edge to WiFI/VOiP (UMA) for Blackberries when no other provider would touch the idea. It's a wonderful thing, as I live on one of those mystical blackholes that eats all cell reception even though 20 ft from my front door they all have 4 bars service.
I pull up to my house and I'm talking on GSM, I get in range of my wireless access point and it swaps right onto it without skipping a beat. Great technology well executed.
They are somewhat sketchy about it though. I travelled back home to the UK and figured it would work on the WiFi where I was staying. A great way to save a boatload on international roaming for voice and Data.
Guess what? It wouldn't work! I suspect that the roaming fees were just to tempting and they were using some form of GeoLocation to prevent the IP link back to them from forming.
Paris, because it's amazing how something that should be so hot and cool actually is lamer than Little Tim.