Yes, actually, it is possible. At least, it is proposed in the serval project white paper: http://developer.servalproject.org/site/docs/2011/Serval_Introduction.html. Serval would help the end user avoid exorbitant fees and bad service areas, but wouldn't help much for running an enterprise service.

[EDIT] It just occurred to me that a serval network would be an excellent hedge against a MITM attack by or using the main cell network. Would have a very positive effect on security. [/EDIT]