Are you by any chance also using the app EasyTether while connected to your Mac? If you happen to use this app, you're in luck, because the solution is to call:
sudo kextunload -v /System/Library/Extensions/EasyTetherUSBEthernet.kext
from a terminal. I forget if you have to reboot or not.
This will disable tethering, but you can now see your device via adb.
To renable tethering once you're done debugging, use
sudo kextload -v /System/Library/Extensions/EasyTetherUSBEthernet.kext
Of course, if you're not using EasyTether, then hopefully someone else has an idea....