There's a company called 2600hz which pretty heavily leverages the use of Freeswitch and provides carrier grade VOIP and virtual PBX services is now offering a mobile option. It's pretty interesting and it is using the backbone of Sprint's wireless network.

Anyone interested in this stuff may want to listen to this interview (It's pretty dry but it works):

and check out their site as well. (Sorry I can't link to the section because of the site. Click the Mobil Integration Section)