I got a Droid X for development, and after setting it up just like all my other test devices, it showed nothing when I ran "adb devices". When I plugged it in each time, OS X complained that it could not connect to the mass storage device.
How can I fix this, and connect my Droid X to OS X for development?
Finally found the answer, so I wanted to share it here.
The Droid X ships with USB mode set to "PC Mode" by default. It needs to be set to "USB Mass Storage" mode.
To do this, touch the USB icon in the status bar and drag it down. Touch "USB Connection" and select "USB Mass Storage" mode.
Thanks for this answer go to EricC (apparently a Motorola employee), found here.
Actually, USB Mass Storage mode usually presents the phone to the host as a file drive. To setup ADB you usually need to go to Settings > Applications > Development and select USB Debugging.