I'm in the same situation. Currently locked into a KDDI 4S, and I refuse to buy another locked iPhone.
If Docomo offers to unlock customer's iPhone's I will be switching over on 9/20 as soon as the stores open.
That said, its Japan, and its the iPhone, so be ready for some BS about how their unlocking policy is for every phone EXCEPT the iPhone.
My advice is to save/print out that page right now (in case they try and change it later), go to their Japanese website, find the equivalent page, save and print that out, and then bring both of them in with you when you go to make the contract.
Be polite, but do not make a contract without a guarantee that they will unlock the phone (make it clear that's why your switching), and be ready to escalate/show evidence of Docomo's official policy if necessary.
I don't know about you,but I'm ready for an unlocked iPhone in Japan, and I'm not taking no for an answer
Best of luck, let us know if you get any more info (I'll do the same)