From April 1, Docomo is scheduled to begin allowing overseas phones, including the iPhone 4, on its network. But there are some catches, potentially deal-killing catches.
First, according to a Docomo press release, there will be some restrictions on which devices can get access to the packet houdai for a reasonable price. Any phone that is capable of tethering (hot spot) will be charged the premium rate, bringing the full price to 10,395 yen for unlimited data. (Along these lines.) Otherwise, a phone can be used with the tiered plan for 5,985 yen for month or the flat rate plan Nicholas announced for 5,460 yen.
Next, there is still the issue of certification (lightning +〒 mark =). Juggly-kun reported (just prior to the quake) that uncertified phones would be allowed but would be charged the premium price.
ちなみに、SIMロックフリー端末で技適マークのある端末（例えばiPhone 4）であれば、パケット定額プラン上限5,985円で利用できるとのこと。技適マークの無い端末では10,395円。技適マークがあってもテザリングは10,395円。技適マークのチェックがドコモショップで行われるそうです。But this must be wrong. According to the press release.
技適マークが付いていない他社携帯電話機のご使用は、電波法違反になるため、ドコモのネットワークへの接続をお断りする場合があります。They could be flat out denied, and I expect they would be. I understand from @havill that phones like the nexus one are entirely comprised of bits and pieces (except for the battery, which can be changed) that are certified for use in Japan. It will be interesting to see how these phones, which don't bear The Mark, will be treated.
Docomo will update their site with information regarding which phones will be allowed, which won't, and which will be charged a premium. In the meantime, if you were planning on installing that update that gives you the ability to tether, I'd hold off if you plan to put your phone on Docomo. If your current Android ROM has built-in hot spot support, I'd suggest to start doing research into ROMs that don't include that feature.
The details in the press release are subject to change, and probably will change. Given the current situation, the entire thing could be delayed, especially if the plant that was supposed to be stamping out microSIMs is shut down to save power.