When I use the ethernet shield & arduino conected to USB, I can send a ping sucesfully. if I try the same with external power suply the ping comand doesn´t works.
The sketch is the same, I´ve tried with Arduino diecimila & duemilanova.
I´ve tried with several power suply from 9V 500mA to 12V 2A.
If I reset the arduino with both connectiones and after it star I disconect the USB cable it still works with the ping. But if I restart only with external power suply it doesn´t. All the leds of the ethernet shield blink in both cases.
Just the green led on The RJ conector blinks faster on the external power mode.
I have a feeling there was some thread on the forums about reset/power supply issues with the shield that were fixed by adding a capacitor somewhere.
Maybe a grounding issue?
Other than not being surprised to hear their is an issue around power, I'm afraid I don't have a fix for you--does it help if you use the reset button after you've connected power?