I wondered about the possibility that the library (or USB microprocessor) is waiting for serial characters. I can't rule that out yet. I tried it with and without the Arduino IDE running, and that doesn't seem to matter, but sometimes I notice that it doesn't work even with this particular USB port. I can try a capacitor from reset line to ground, maybe...
Aha, I made some progress! If I plug it into the wall charger, and then hit the reset button on the Ethernet shield, it works! Hmm, what does that tell me?