Cellular-over-cellular?

Here's a random thought.... UMA/GAN is intended to offer GSM services over a "generic" IP connection.

The usual type of IP connection implied is WiFi from a dual-mode phone, (linked back via ADSL or cable broadband), with the occasional comment that "it'll work with WiMAX too when it arrives". I've also had a couple of people suggest using fixed-line VoIP telephones hooked into a cellular core network as well.

But how about another type of IP "pipe"..... a cellular one, ideally another carrier's. How about GSM-over-3rd-party-WCDMA? Or even more entertainingly, GSM-over-3rd-Party-EVDO?

I haven't even begun to think about the business models here, or the practical difficulties in implementing this, but given that there are a growing number of GSM/CDMA dual-mode devices, presumably it's only a matter of time before somebody tries something cute with them....