In particular, I'm seeing a fair amount of mobile operators use their own, or partners' networks of public WiFi APs. Most notably, AT&T acquired Wayport and has other footprint for use in iPhone offload, while Vodafone has cut a deal with BT OpenZone in the UK.
However, there is one problem as the density of such offload points increases. As well as the main public OpenZone points in London, I can also use the BT Fon "virtual hotspots" as an extension of my home broadband account.
While this is great for my laptop, it's causing me difficulties on my smartphone. The density of BT / Fon / OpenZone access points in central London is so high (in homes and offices or other locations) that as I walk down the street, my iPhone keeps attempting to register. But by the time I've got online, I've walked past it and onto the next one along the street with stronger signal.
If I walk 5 minutes from home to my local tube station, I need to switch off WiFi temporarily, if I actually want to use mobile data - otherwise I have a constant stream of pop-ups from the connection manager on-screen, and no reliable connection.
It probably wouldn't help with a high density of femtocells either, until there's a reliable way of doing femto-to-femto handoff as you walk down the street.
Food for thought.