A company called LongBoard has developed software for Wi-Fi and cellular enabled phones that enables voice on both networks: It hands off calls between the networks, but not in the smoothest fashion. When the phone gets near the edge of a Wi-Fi network, it dials the call on the cell phone network. The caller actually talks over both networks until the Wi-Fi network drops out completely. I'm very curious about the voice quality as both calls are operating simultaneously.
A couple of small handset makers have agreed to include the software on their devices. The software can also work on Symbian and Pocket PC devices, although apparently it's not very reliable on PocketPC.
Many companies I've spoken to say handoff between cellular and Wi-Fi networks isn't that difficult. The hard part comes on the backend with forming business relationships between operators and settling billing. If it is relatively uncomplicated to do real handoff between the networks as they say, it's not clear why LongBoard has developed a less-than-ideal solution. It could be that the LongBoard solution is all software and others aren't, which could make it easier for LongBoard to sell to handset makers.