I'm regularly asked to explain my scepticism of UMA and so-called "seamless" behaviour of dual-mode WiFi/cellular phones.
One of my core beliefs is that consumers will use WiFi-enabled mobile phones in a very different fashion to ordinary cellular-only handsets. They will want the WiFi function to do a lot more than simply extend cellular coverage, and offer transparent "access agnostic" links to existing mobile services.
In particular, I think consumers will want to do a lot more than access "services" with a dual-mode device. They'll start treating the phone as a piece of home consumer electronics, rather than purely a "service-led" device. They'll want to get MP3 files from their PC's hard drive over WiFi, maybe share contacts, and possibly hook into all the other new bits of WiFi-enabled gadgetry around the home. They'll want to use the phone to browse the web via their broadband connection from the sofa (with no per-MB charges), and use the integrated WiFi to access email, IM, VoIP (yeah, maybe Skype) and all that other good "proper Internet" stuff.
UMA handsets, on the other hand, will tend to usurp most of this in favour of billable operator services, just using your WiFi as a way of tunneling the (locked, walled & expensive) cellular "user experience" over broadband. On some phones you might be able to get at some of the other functions over WiFi, but only if you go down 17 levels of menus & obscure configuration settings, and your operator hasn't locked all that stuff down.
On the other hand, UMA and its next generation, 3GPP GAN, is actually a standard, while the various approaches using SIP alternatives are more proprietary, and also tend (at the moment) to lack the much-hyped voice seamlessness which I think really isn't that big a deal.
All of which means I was very happy to find that French broadband operator NeufCegetel appears to agree with me, and is working on a trial dual-mode solution that uses SIP and a proper PC/Internet-integrated smartphone. "Choose music, photos, videos or other documents on your computer or the Internet and load them at high speed"....
.... although maybe, just this once, they should have diverted a few pennies from their clearly very innovative R&D team, and spent a bit more on their marketing & branding efforts.... I mean, I know beauty is in the eye of the beholder, and the Qtek 8300 that NeufCegetel is using isn't really an UglyPhone, but still, would you want to say you had a "BeautifulPhone"?