If my input is of use, here is my suggestion.
As most devices are gsm for the most part, with a cdma counter part, I think all devices should have a single forum.
If / when a cdma version is released, either link into a cdma sub, leaving the general as general/gsm or just have 2, one for each architecture. I think carrier branding device forums should be avoided as much as possible.
Only major deviations in architecture should warrant a sub / alternate forum.
That's what id do. Id go for the first though, as general stuff will fit both but keep it in one place. Accessories, issues with gfx, etc