I'm seeing the same behavior with my older 512 mb iPod shuffle -- it was working fine one day, the next neither Mac OS X (on three different computers) nor Windows XP (one machine) can see the device. It does not mount on the desktop, nor do any of the indicator lights flash to tell me that it's connected or even charging. My theory is that the USB connector must be shot; the shuffle itself was still able to play until it finally lost its charge.
I tried resetting it several times, but no dice. Now that it's charge is gone, I tried connecting it again, but alas, still nothing.
AAM: Shuffle Problems