There was a discussion of integrating Wi-Fi into cell phones at the Communications Design Conference: Some analysts think we'll see the devices in production by the middle of next year.
A lot will have to be worked out before a combined cellular/Wi-Fi voice service can be aptly offered. The only way that will happen is if the cell carriers can figure out a way to control the air time that their customers spend on the Wi-Fi networks to ensure that they continue to make money.
There's a company here in Seattle called Bridgeport aiming to solve that problem for cell operators. A cell carrier would put Bridgeport's box in its mobile switching center. If a customer is within range of a Wi-Fi network, the call is routed to the carrier's mobile switching center like a regular cellular call. That way the carrier can track the usage on the Wi-Fi network and charge customers.
It's not an ideal setup, but I think carriers are going to require something like this so they can control and charge their customers for using voice on Wi-Fi networks.
This story in EETimes also covers the discussion of Wi-Fi and cellular from the conference: But the reporter also talked to Craig Barratt of Atheros about smart antennas. It sounds like there's a lot of work going on with smart antennas and Wi-Fi but a TI technical guy noted that his tests so far showed that smart antennas don't do much for Wi-Fi.