HTC have made available (finally) the official unlocked bootloader via the htcdev.com site. OK so the HTC Desire is a fairly easy phone to hack and replace the locked bootloader using alpharev and to install root access using reflash. The new replacement for these tools is revolutionary, this simply combined the two tools into the one.
So the latter of the above is the unofficial way and for most (including myself) have used to replace the locked bootloader, however now the tool from htcdev site gives you a link to download a replacement bootloader (that is locked at this point) Now this is where for me the expectations from the bootloader program didn't agree with both the version of hboot and current android version.
The bootloader program (naturally only runs under MS Windows) that you download from htcdev you need to select your carrier and the firmware update number (froyo)
So here's the catch, the RUU that you download is expecting to see the release of hboot that came with the official froyo update. So if you have any other hboot version then you are going to get an error stating that this update doesn't support the release (number) and that an update will be released soon. Clearly the latter is untrue! but then I suppose it's better than simply being told nothing! So it's essential to restore the hboot version that is included with froyo. It's also necessary to roll back to the official froyo release (don't worry you can remove it once you're done)
Once you have restored froyo you can go ahead and run the RUU to replace the hboot version (yes again!) It's now that you can follow the instructions on htcdev to unlock the bootloader!
Ok so I decided to go down this path of using the official unlocked bootloader, however in all honesty it's far easier to use the alpharev bootloader!