UPDATE: per a comment, the possibility has been raised that, even though this phone is unsubsidized and unconnected to any carrier, the settings may still cause problems with tethering on MVNOs. The XML preference file defining an APN has an option to set whether it allows tethering, and it could be possible that there is no way (without hacking the phone) to allow tethering with an MVNO.
If so, sorry iPhone lovers, but this is a total defect in iOS.
Well, that's a surprise.
iPhone 5S and iPhone 5C store pages are here.
You can take this to any Japanese mobile carrier and theoretically use it, but KDDI doesn't sell just SIM cards for unlocked phones, Softbank won't sell LTE SIM cards, and emobile doesn't have nano SIMs (so you'll need to chop down the SIM card). NTT Docomo will sell you a SIM card for this no problem, but you won't be able to get a docomo email address because sp mode can't be used with phones not sold from docomo.
I'd suggest to get the b-mobile "Free Data" SIM, which went on sale just today. Nice timing.
I turned out to be wrong that the docomo iPhone was locked out of MVNOs, but it cannot be used to tether with an MVNO (because the utility for creating the Carrier Profile won't allow an APN to be set for tethering). So, this unlocked one is the best way to get full iPhone functionality with an MVNO. The FAQ says that unlocked iPhone can do tethering.
Shipping times are showing one to two weeks for all colors and sizes, and the price ranges from ¥71,800 to ¥91,800.
As far as models go... I don't know which one is best. I guess it depends on where you tend to travel. Just don't get the CDMA versions.