For now, the N1 can be used in Japan, but only on softbank. Unless someone absolutely needs to purchase a new phone now, I would wait a few months to see if Docomo does indeed allow foreign phones to be registered and used on its 3G network. Commenters report getting much slower data transfer rates on a docomo HT-03a when switching it to softbank.
An xda member and sometime commenter on this blog had this to say on the subject of using the N1 in Japan.
I can confirm the Nexus works fine on Softbank, for people with black Softbank sims with unlimited iphone data. The black sim world.smile APN details are easily google-able. One comparison I made though, Softbank browsing and internet speed was so terrible it did cramp the Nexus a little. Browsing and data felt slower on the Nexus on Softbank than the 03a on Docomo, despite the Nexus being a much faster handset.Other commenters on this blog have mentioned that SIM cards for the X-series softbank smart phones will also work with Android phones and that it is possible to walk into a store and apply for a plan, but no one has commented directly on using these SIMs with the N1.
Also, keep in mind that the APN for the X-series gray/silver SIM cards is different from the APN for the iPhone with the black SIM cards. Both require a subscription to the packet flat-rate full plan for the iPhone and X-series phones. DO NOT, under any circumstances whatsoever try to use a softbank SIM if you are subscribed to the regular, unlimited packet discount plan for non-smartphones because very unpleasant things will happen.
One last thing, not that I should be pointing out typos and grammar mistakes as this blog is littered with them, but anyone else notice the that on the English flat-rate plan page at softbank, iPhone is misspelled?