It's not a new idea, but it's starting to percolate down: Travelers might use voice over Wi-Fi instead of very expensive cell long distance when roaming. (I've toyed with buying Vonage service, and bringing the box with me when I travel so I can make unlimited calls and receive incoming calls when in a hotel room or anywhere I can bridge to a wired to Wi-Fi connection.)
Check out Pulver's FreeWorldDialup, http://www.freeworldialup.com/
Another pretty interesting try to make a seamless use of WiFi networks on handhelds and notebooks is Telesym (http://www.telesym.com). Quite a fancy product!
Voice-over-WiFi has great potential when combined with emerging technologies such as Vivato's long-range WiFi switch. While there are problems with quality of service on any shared medium network, VoIP for the most part provides much better quality of service than cellular. And at a tiny fraction of the price, what more do you need than a good quality client device?
Check out xten networks. Their SIP softphone works great over WiFi.