I found this company, MobiBoo while going through news items this AM.
I'm rather impressed by the array of services they want to offer, as it shows they have thought through the approach to Mobile VoIP as a cellular replacement until Fixed Mobile Convergence comes along en masse.
This type of offering is perfect for Boingo and Earthlink to consider as they are ideally suited to work together and could with Helio take things to the FMC level. But I also feel as a staunch believer in Muni Wireless that as more hotspots get built out that are metro wide, that services like this are going to be more robust.
Go through the MobiBoo web site and see how they have thought through their offering. The idea of a Mobile VoIP PBX, plus the soft client options, yes option, they have two is stroke of genius.
All they are missing is a converged device, and I know of one or two that are almost ready for prime time to do just that.