On Fedora, I was having issues getting my T-Mobile G2X to connect in developer mode.
For sure, I was setting Settings-Applications-USBdebugging to Checked. And when plugging in the device, the SD card (and/or) internal memory space was showing up on the desktop (and the output in /var/log/messages looked equivalent to that from my working G1) - but no joy with : adb devices.
The Fix : Eventually, after trying everything I could think of, to no avail, I finally toggled Settings-Applications-USBdebugging off-and-on while the phone is connected via USB. Works fine now :
$ adb devices List of devices attached emulator-5554 device 0288NN0NN2ffb517 device
In fact, it works rather too well, since I can't go back to the old non-working situation to check that that was really the fix. The step I took just prior was to cancel the Service for Car Home. But that's running now, without causing any problems. Very strange.
Please leave a comment if you have more information that can help prove/disprove that this is the right fix.