Please understand that the source code to the GSM modem that you don't have access to, is precisely the portion of the iPhone firmware that is being manipulated by the carrier unlock procedure (but afaik not the jailbreak procedure). Jailbreaking, however, is the gateway to the unlock process â" hence the concern about prohibiting Jailbreak.
Every version of modem/phone baseband firmware must pass significant interoperability and carrier approval test suites before it can be sold or distributed. Why? Because cell networks are heterogeneous amalgamations of multiple vendors handsets, basestation equipment, and firmware versions for each. Think its annoying when a cheap WiFi router doesn't interop well with your existing devices? Is it WiFi certified? Now imagine being a network operator when potentially rogue firmware versions randomly take down portions of your billion dollar network...
Should the unlock process accidentally or otherwise modify the modem/phone calibration or call-handling procedures, the network can be impacted. The cell network stability is dependent on all handsets following the same rules â" one misbehaving handset can affect the rest of the cell... Imagine having your call dropped because someone running an unlocked iPhone did a handoff to your cell...
e.g. One early iPhone unlock procedure caused every unlocked device to have the same IMEI â" an identifer which should be unique in the world. This can cause network (and billing) issues analogous to multiple Ethernet devices with the same MAC address.
Thus Apple's concern about unlocking (which required jailbreaking) may be motivated by customer support concerns.
Should poorly unlocked iPhones accidentally start affecting cell basestations (perhaps only one manufacturer's basestation), Apple will be faced with customer support calls for firmware that didn't pass the standard certification processes, the network operator will be faced with customer support calls, and immense finger-pointing will ensue (between the network operator, Apple, customers, and basestation manufacturers). Reputations will be tarnished and Apple's customer support costs will jump, even though the Apple QA'd firmware does not have the same defects as the hacked firmware. Revenue and expenses will be impacted.
Stepping back and looking at the economic motivations, Apple often makes decisions based on the desire to minimize costs â" customer support costs.
(this also ignores lost App Store revenue, carrier kickbacks, etc. from jailbroken and unlocked iPhones)
I'm all for the freedom of unlocked and jailbroken iPhones, but accusing Apple of trying to maintain a closed system just because they're jerks is ignorant of the processes in place to ensure network stability and a positive user experience by all cell phone users.
(No, I'm not accusing the iPhoneDevTeam of maliciously attempting to release poorly-hacked firmware, but given the earlier IMEI fiasco, it IS possible. I expect we all believe in QA processes, and I doubt the DevTeam has access to a Spirent
Its way outside their budget. )