The significance of WiFi-cellular roaming doesn't lie in cutting voice costs. (The picture, by the way, comes from Novinky, a Czech online magazine, a story about DSL.)

The significance of WiFi-cellular roaming lies in Always On applications.

Think about it. Cellular channels are relatively low in bandwidth, WiFi channels are high in bandwidth.

Now, you're wearing an application, like a heart monitor. When you're at home, or in your office, this thing can be generating, and immediately disgorging, tons and tons of data, detailed stuff that may be fun for your doctor to analyze later.

So now you go to the store, or somewhere else. You're still wearing the same shirt, but now you're in a low-bandwidth environment. Now that data could be saved, to memory chips in the shirt (the algorithms could compress this data automatically if you're out of range a long time), and a call would only be placed if an emergency condition exists.

All the necessary chips could easily be embedded in your UnderArmour. (Or some other brand -- I just think UnderArmour is cool which makes the application easier to sell.) Or the whole unit (no bigger than a wire) could be installed with no more thought than a tie (so you can clean the shirt - even UnderArmour will stink eventually).

The point is that medical, automation, and security applications for Always-On can now be both mobile, and more data intense, simply through the use of dual-mode radios.