[Upgrading, Modifying and Unlocking] axel007:
For posterity. The recent Softbank Desire release (after August, far as I can tell and with Android 2.2) is a little different from most models in that it does not allow unlocking. When inserting an unauthorized SIM card the phone merely gives the emergency dial pad but not a prompt to unlock. The only way I have found to unlock the phone anyway is to first flash a different ROM. This may not be acceptable to some of the less technically interested users in which case it looks like you will not be able to unlock that phone (but I would be very happy to hear from anyone who has a less laborious method).

If you are willing to flash a new ROM, then it will work. I got the unlock code from GSMliberty. They had to charge me twice, once for the standard database search they do, and then again for the expanded database search with the newer codes. Whether or not this kind of up-selling is a scam I don't know. But the code I finally got did work. There was one slight hiccup after entering the unlock code, namely that the phone asked for another "unlock subset PIN". I had no idea what this was but tried with the same unlock code again and that did it.

I must say that I also found the instructions available here and on other boards for the entire rooting and flashing process insufficient for someone who doesn't know Androidspeak, so I compiled me own tutorial. Here it is.

How to install custom ROM on Softbank HTC Desire (with a Windows Vista PC - other Windows versions are similar). IMPORTANT: this will void your Softbank warranty. If you need service from Softbank based on the warranty, you will need to reverse the entire below process. Which is why you must back up your stock ROM. ALSO IMPORTANT: I give no warranty for this tutorial and will be unlikely to give any after service in case you hit problems)

This tutorial includes the use of ROM Manager, a paid application that makes a few things a little easier. One thing it does not do is allow you to use ROM Manager for the initial flash because again, Softbank's stock ROM somehow manages to sabotage ROM Manager.

Create Google Mail Account if you don't have one (you will need this to buy ROM Manager from the Android market)

Install HTC Sync on your PC (which you can download from the HTC website) for the included drivers, then uninstall only the Sync application itself, leaving the drivers (they appear in the usual Windows software uninstall list)

Install HBOOT Drivers from here http://xdaforums.com/showthread.php?t=788044, then follow the instructions from the same page to install Unrevoked3

Set up Android Market account on the phone (need credit card and Gmail account)

Download the free ROM Manager app and install

Buy the premium ROM Manager app and install

Allow / initiate install of Clockwork Recovery (which is an application somehow associated with ROM Manager)

Flash Clockwork Recovery (ROM Manager gives an option to do so)

Using ROM Manager, create a back-up of the stock ROM

Shut down the phone, take the SD card out and insert in PC; find /Clockwork/Recovery and copy the folder onto your PC (should look something like "2010-12-29-11.09.26"). This is your backup of the Softbank stock ROM. Don't lose it.

Put the SD card back in the phone, restart the phone, and create external partition on SD card, again using ROM Manager (512MB or more if possible)

Again shut down your phone, take the SD card out and plug it in your PC

Download your chosen unauthorized ROM and Radio onto your PC (I got Neophyte 8.2 which includes Japanese language support), copy the zip files to the root directory of the SD card (root directory means the first level, not any of the folders therein)

Reboot phone with SD card into Recovery (from a switched-off phone, hold down Volume-Down key and Power until the menu appears)

Go to Recovery, perform a cache wipe and a factory wipe/reset

Again in Recovery, using the option for installing zip files, find the ROM and Radio files and install them successively. Reboot. The process of flashing these two packages will take a grand total of 15 minutes or so. Do not touch your phone until it shows the normal Android home screen.

When phone is working, next apply the unlock code as per instructions from the vendor

Reinstall ROM Manager for future ROM flashing

You're done!

Bonus action: look for an application called Titanium Backup (which is included in some of the unauthorized ROMs) to back up your applications and settings for after the next ROM flash you do