Sounds like a fun/interesting project to take on.
If the SIM is accessible from the OS on the device, then it seems that it would be possible to build an implementation. What is unclear to me (but may be cleared up reading the relevant standards) is if the call is just data on the cellular side. If it is, then moving from wifi<->cell should be possible. If not, then I suspect the hardware would require some special support for it.
Either way, I wouldn’t expect it to be included in the initial release. My understanding is that the initial release will be just the basic functionality needed to call it a phone. (Think Android 1.0, or possibly less.)
One, possibly interesting, follow on question. Has anyone looked at the source for the Ubuntu phone to see if it supported UMA/GAN? That might direct us to some existing open source software that implements the standards, which would make supporting it MUCH easier.