EDIT: Here is a link to discussion of this ROM on XDA. (Stevo on CM forums = amazinglarry311 on XDA.)
Keep an eye on this post, as I'll be updating as I use this ROM. It comes with Launcher 2, but I plan to replace that with ADW pushed to /system. It also has no support for the Japanese locale, so I am going to be working on adding that as well. Depending on performance, I may flash swapscript 2.1.
CM6 nightly builds, which can be found here for the ht-03a.
Here is the first ROM based off CM6 and the T-Mobile official Froyo update to the My Touch 3G, which is the same phone as Docomo's ht-03a. A full wipe of the phone is required first. Do a nandroid backup first! I'd also suggest using Titanium Backup to backup your user applications to save the hassle of having to download them again. Both Simeji and ADW launcher have options to save your settings to the SD card, which is good to do so that you can quickly restore.
The official update is coming in two parts, the first part of which seems to contain an SPL and radio. It appears the SPL is "perfect," meaning that anyone who installs the update from T-Mobile with have to use a goldcard to change SPLs if they ever wanted to use fastboot commands. (Note: the ht-03a shipped with a perfect SPL.)
The first step for anyone who has an engineering SPL is to erase /system /data and /cache with fastboot. (This will not work for anyone who used Universal Androot on the ht-03a. Those people should flash the zip in main post to erase. This can also be done by booting into recovery, but does not always result in a full wipe and is therefore not the preferred method).
fastboot erase system -w
Next, flash the zip from recovery and reboot. Go to Settings/Wireless Networks/Mobile Networks and deselect roaming if it is on - don't think this will matter in Japan, though.
So far, it does seem quite speedy, though for me, it is not amazingly fast compared to CM6. I think the people who are absolutely freaking out about that, for whatever reason, had an exceptionally slow experience with CM6. I'll update below as I use this more.