Interesting announcement from Trapeze Networks and a new company Divitas, the latter a company I hadn't heard of before. A quick scan of its website & a few articles like this one and it all starts to make an awful lot of sense.
Conceptually, it sounds like a similar product to Motorola's Wireless Service Manager and also fits with some of the activities that Nokia's enterprise division is working on with Cisco and Avaya IP-PBXs. Basically, there's a smart controller box in the enterprise network and a client on the handset. The carrier is more-or-less relegated to being a pipe (completely the right approach for large enterprises, in my view), with tight integration with the enterprise-grade WLAN network (something that is totally absent in UMA and most forms of carrier-based SIP mobility management).
The main problem that Divitas is going to have is ensuring testing and interoperability with the myriad software platforms that dual-mode devices will come in. Doing a Windows Mobile 5.0 port on a dedicated handset is fine - but they will also have to work with countless other WiFi-enabled devices that corporations & their suppliers prefer. In some cases, these will come in operator-specific versions that may even try & prevent this type of usage.
Divitas is also going to have work hard on its channel to market. In my view, this type of dual-mode solution will have to fit in with the enterprise telephony world, where the IP-PBX vendors and their resellers are the kingmakers & gatekeepers.