No receiving of MMS or 3G video phone calls if you have a system like this. Won't be an issue with N900 for now at least, though I love the idea though, i've been hoping for something like this to extend battery life.

@revdkathy, @steve4130 do you really need to manually switch between 2g and 3g modes, i would assume all phones know how to down to 2g if 3g is unavailable.