UPDATE: It appears that AT&T is using AWS for LTE, which is Band 4. Docomo Xi is using Band 1. If Apple is referring to Band 4 when it says "2100", then this likely won't work. If it is referring to Band 1 and Band 4, then it should.
As far as I know, NTT Docomo operates the only LTE network in the world at 2100 MHz. However, this doesn't mean that a Docomo iPad is coming. Docomo's and Apple's ideas for branding and marketing appear mutually exclusive. There's no way that Apple is going to allow Docomo defile their product with labeling and preinstalled junk, just like Docomo isn't going to agree to the terms regarding retail display, licensing, etc. that would be likely forced on them by Apple.
However, an unlocked version of this next iPad would likely be compatible with an Xi microSIM, but don't run out with that in mind and buy one just yet. Let's see how this shakes out. The SBM version will likely be SIM-locked in Japan.
If Softbank Mobile didn't win the 900 MHz allocation, it would have greatly hindered their plans for roll out of an FDD-LTE network, presumably at 2100 MHz. At some point this next iPad will probably be compatible with Softbank LTE, but by the time that happens, we'll be looking back at the release of the next iPad after next, or whatever they call it.
Regarding AU, their plans also include 2100 MHz, but like US Verizon, they are a CDMA-2000 carrier. Note that the "Verizon Model" has no indication of 2100 MHz.