A group of companies that have banded together to work on mobile voice over IP will be demonstrating handoffs between GSM and Wi-Fi voice over IP networks at the 3GSM conference: BridgePort, IBM, PCTEL, and Verisign are some of the companies involved in a group called MobileIGNITE which is supporting this type of roaming.
These companies are supporting Session Initiation Protocol (SIP), a commonly used voice over IP interface, to deliver voice over Wi-Fi. It appears that these companies are mainly targeting enterprise or government users. So the end users might have a combined Wi-Fi/cellular phone and with this solution, they could use the handset to make and take voice calls over their corporate WLAN. This is different than the companies involved with UMA, which allows roaming between Wi-Fi and cellular networks but relies on existing components of the cellular network in order to allow for authentication and billing and other services.
I've long thought that while the idea behind what MobileIGNITE is doing is good, the mobile operators will resist it. They would much rather support Wi-Fi/cellular roaming the UMA way, where they have the most possible control over the call from end to end. They want to charge for every voice minute, whether the voice call is handled over Wi-Fi or GSM. But from what I understand about Bridgeport's model, the MobileIGNITE companies want to offload voice traffic onto the enterprise WLAN because it's cheaper for the company to support those minutes than pay the GSM operator. Enterprises will have to work out the cellular/Wi-Fi roaming with their mobile operator in order for this to work and that negotiation is one that I think will be difficult.