adb devices list empty -- Snow Leopard

I'm running into an issue with the android SDK where it doesn't recognize devices plugged in via usb:

$adb start-server                                 
* daemon not running. starting it now on port 5037 *
* daemon started successfully *
$adb devices                                      
List of devices attached

(list is empty, other sdk tools don't recognize the devices either)

According to the SDK documentation, on Mac OS X, this should "just work". (Setting up a Device for Development)

It's worked for me in the past, but seems to have just stopped. Is there an OS X solution to this problem? (On other operating systems presumably you have to fiddle with USB drivers)

Have tried:

Power cycling machine and device
Switching USB ports Unplugging all
USB except for the sole (non-split)
cable to the device Multiple cables

Solution

Another thing for people searching who are still stumped (as I was for some time)...

I had this exact same issue and just figured out what helped for me. I had installed EasyTether and it was interfering with this even after I thought I uninstalled it.

Here's the solution from the EasyTether website:

ADB on Mac OS X stops recognizing the attached device after installing EasyTether driver. Use kextload/kextunload to unload from memory the EasyTetherUSBEthernet.kext kernel extension manually. It is in /System/Library/Extensions/