I think your main problem would be type approvals, especially as this involves telecoms and wireless. Whatever you intend to make, has to be FCC approved in the US, CE in the EU, then there's UL, GSM Association etc, also many other contries their own various regulations and rules you'd have to deal with. Also you have to have IMEI numbers. Probably at quite considerable cost. GSM Assoc membership alone could cost you over $50,000, which AFAIK
you must be a member of if you wish to make cellphones.
These things are never a problem if you're the likes of Samsung, LG, Motorola, Huawei, ZTE, etc. but for a hobbyist or community of devs and hackers, who wants to make their own cellphones, it's certainly a problem. The nearest thing you're going to get is building custom ROMs for exisiting cellphones, because they're already approved, and you're not doing anything with the electronics, radios, IMEIs, MACs, batteries, chargers, etc.
I'm sure if a phone doesn't meet whatever required approvals, no carrier is going to touch it, plus it would be illegal to sell and use as well. If you haven't got an IMEI number, it's not going to work anyway.
I think the difference is between making a cellphone and building a PC, is that most of the parts like mainboards, PSUs, hard drives, etc are already type approved by the makers. And when you "build" a laptop you're not really building it, you're fitting a HDD and RAM into what is a pre-made computer.