If you have installed EasyTether on OS X, the EasyTetherUSBEthernet.kext can prevent adb from communicating with your mobile device.
If you use Easy Tether, BEFORE you plug in your phone run:
sudo kextunload /System/Library/Extensions/EasyTetherUSBEthernet.kext
(or unplug/plug your phone in after)
If you are done debugging and want to use EasyTether again, either restart or:
sudo kextload /System/Library/Extensions/EasyTetherUSBEthernet.kext
If you want to remove the EasyTether kext so it never hassels you again:
sudo kextunload /System/Library/Extensions/EasyTetherUSBEthernet.kext
sudo rm -rf /System/Library/Extensions/EasyTetherUSBEthernet.kext
Reference/Credit: http://www.intohand.com/post.php?s=2011-02-17-android-debug-bridge-not-just-working-on-mac