Mobitus, the VoIP side of FatPort, one of Canada's top WiFi operators, has added VoIP with login using the Zyxel Presitige handset according to WiFi Networking News earlier in the week.
I've used the Zyxel and have not been blown away with the sound quality at all. I just received from Net2Phone their version and should see the Broadvoice version next week. Each company has done something to the firmware, so the generic Zyxel that I have configured to work with either Vonage or VoicePulse may not be as true of an indicator of what it can really do.
The bottom line is VoWiFi is a coming thing. I prefer though a softphone and laptop in the coffee shop environment, but have found that the WiFi phone is very useful when I go to friends houses that are WiFI enabled so my phone is with me when sometimes cell service isn't.