As far as I see it, the most interesting potential outcome of a Docomo iPhone is enhanced unlocking of phones between Japan's two large W-CDMA carriers, Softbank and Docomo. This will eventually encompass KDDI as Voice over LTE replaces 3G. (KDDI is currently immune to unlocking because they use an incompatible CDMA-2000 network for voice communications.)
Recap: The origins of SIM unlocking in Japan
SIM unlocking (as it's referred to in Japan) started several years ago, following Softbank Mobile's successful introduction of the iPhone 3G. NTT Docomo found itself on the defensive, for the first time ever, and you do have to sympathize just a little with the the arrogant bastards. Survey upon survey told them exactly what people wanted in a phone: mobile payments, water resistance, infrared, solar panels, one seg, pedometers, labyrinthine mazes of menu options, and an eyebrow plucker. The iPhone had none and this, Apple had no experience making phones, and Macs held a dismal share of the PC market in Japan.
And yet the iPhone quickly achieved unprecedented popularity in Japan, prompting Docomo to attempt to sidestep Apple by successfully pushing for the introduction of SIM unlocking guidelines.
(See our coverage of the debate, including an overview plus the opinions of Docomo and E-access, KDDI, and Softbank.)
"Guidelines" as set forth by the the Ministry of Infrastructure and Communications are just that - they are voluntary and not "requirements." However, in practice, telecommunications entities followed them. Up until now. Softbank simply refused to comply with the unlocking guidelines, even following contract completion (and even as they were buying back phones, unlocking them, and reselling them in emerging markets).
And so we fell into a stalemate.
KDDI and E-access (Emobile) are, for various reasons, not relevant to the SIM lock debate; it currently revolves around Docomo and Softbank. Softbank holds the coveted iPhone and no offering from Docomo has proved popular enough to make unlocking an issue to enough consumers. Here's why I think this will change if Docomo sells the iPhone.
Implications of an unlocked Docomo iPhone
Softbank phones are not compatible with the budget data and voice plans offered by MVNOs because Docomo provides the infrastructure. This means that only unlocked phones (or Docomo phones) are compatible. Several years ago, this would not have been an issue, but the MVNO space, led by Japan Communications Inc, has seen substantial growth.
(Comparison of all Japanese MVNOs.)
Many household names such as OCN, Panasonic, and Rakuten have entered the fray, lending legitimacy, in the eyes of the average (non-techy) consumer, to the notion of mobile phone service provided by an entity that is not an actual mobile phone carrier. This results in a higher intrinsic value for a device that works with Docomo versus one that does not (all other things being equal).
If NTT Docomo does indeed carry the iPhone, and if they continue with their tradition of unlocking all their devices, then this would create a large feature disparity between Japanese iPhone carriers. Any discrepancy in value is amplified by the fact that, once you wade through all the fuzzy math to arrive at the out-of-pocket cost of hardware and service, the total cost is almost always the same for similar devices across competing networks.
Recall that previous iPhone feature disparity, with respect to tethering and data caps, left Softbank on the brink of a customer revolt. This was narrowly averted by a yarimasho (aka executive order) from CEO Son. Softbank had no choice but to offer free tethering and ease data caps to match the value offered by KDDI. (Microsoft should have seen a similar reaction coming with the Xbox One versus the PS4.)
Unlocking phones is a slippery slope for carriers. I believe that NTT Docomo will, after making such a stink about unlocking and vowing to unlock all their phones, have no choice but to unlock any iPhone they may sell, just as they currently do for all Android device. If this creates a strong enough feature disparity, Softbank will be forced to follow, and they would lose any justification for keeping their Android devices locked.