During development of the Magic Baby Books application, we tested the application on a HTC desire. Problems were encountered when connecting the device to a computer and it wouldn’t appear when the ADB Devices command was called. When developing for other mobiles we did not encounter this problem, but the HTC Desire ships with its own HTC Sync drivers that are not included in the Google Android SDK.
After some headbanging, a tutorial post by James Giang titled ADB Drivers for HTC Desire came to our rescue. The solution was to download and install the HTC-Sync drivers, and then reconnect the device using USB. Thankfully James has made these available, and they can be downloaded and installed manually from here: http://dl.dropbox.com/u/1349362/HTCSync_2.0.33.exe
After installing the HTC Sync drivers, reconnect the device in Disk Drive mode and the ADB drivers will be successfully installed. Your phone should show up when the ADB Devices command is run. Thanks James!