Aloha 53399,
The exact same thing happened to me. I started a trial account with .Mac to be able to keep the @mac.com email address (assuming I'm able to convince the wife that we really need MobileMe, that is), and at least in System Preferences, I can access MobileMe using my username and password. On the web app side of things, that's "where it all goes sideways" (a quote from The Bourne Identity). The password is not accepted, and I get the same Apple Store Japan reference after I change it back to its initial value. What's even screwier is that in Mail.app, where I also have my MobileMe account set up, I have received at least three emails informing me that my password has been successfully changed. Everything works except for the web interface.
I called my local Apple Store and they referred me to 1-800-APL-CARE, so I'll be calling them later this afternoon to see if I can get this sorted out.
HawaiiMacAddict