My info is that an old bluetooth stack has been integrated into Android 2.2, and it appears that HTC have not upgraded this on the Desire. Therefore there is no HID functionality.
I believe that this has been achieved for other phones - The Dell Streak (Android 1.6) for example works perfectly with the Logitech Mini-Novo kb. whereas my Desire does not.
I also bought a Freedom Pro kb. with the same thought as Shofar, but can only get it to do simple text input when kb. set to SPP mode. The current setup possibility is limited to text input as many of the function keys cause the driver to stop.
This is the biggest downside to the Desire IMHO. Hopefully Gingerbread will deal with this issue.