adb devices was working fine last year, but after upgrades and new software installs,
adb devices no longer recognizes USB attached Android devices.
Her
I just ran into this problem using my HTC Inspire (Android 2.3.3) and Mac OS 10.6.8. The phone did not enter USB debugging mode when I connected it to my USB hub, but it did start USB debugging mode when I connected it directly to my MacBook Pro.
... and then, of course, it showed up in the list of devices attached.