The regular IMEI-generated lock codes worked to unlock HT-03a handsets running Android v. 1.5, but no longer work for v. 1.6. The OTA update pushed by Docomo also relocked unlocked handsets.
Via The Blog That May Suddenly Disappear, we found a company that claims to be able to unlock Docomo's 1.6 firmware... for about $100US! (¥10,000, actually) It takes three days. If you are running 1.5, they'll unlock it for ¥8,000 and then upgrade it 1.6.
At least that is what we think they are saying. Or maybe you'll still need to pay ¥10,000, since it seems they would have to first upgrade to 1.6 and then unlock. My guess is that they will not flash the Docomo 1.6 ROM onto it, but some more standard ROM.
Actually, now that I think about it, that is probably what they are doing to get 1.6 unlocked in the first place...
Anyways, the site is an awesome retro, circa 1995 thing that I am surprised doesn't make use of the (thankfully) deprecated <BLINK> and <MARQUEE> tags (which we resisted the urge to use here), though it does have some blinky flashy animated gifs. They also provide a machine translation "hear."
Use these guys at your own risk!!