Well... We are in something of a rising world of DIY projects. Maybe some deal could be reached with a carrier where kits and compatible parts are sold to the consumers, but to be used on a network, a special activation fee to the carrier has to be paid, and the carrier covers costs like gsm assoc membership in return for those fees.
Or even start with kits for wifi-only tablets, see if there is interest? I could see it catching on a little more with tablets than phones, because they wouldn't need the ruggedness of phones, so assembling them would be easier.
I would buy a kit if this type of product were out there. It'd be fun to put it together, and work a little more hands on with Android and kernels. I also think it would spark other Linux based mobile operating systems to be brewed up by devs.