I purchased a Ping unit yesterday and I am having some issues. If the circuit is powered via the Duemilanove on uSB, all is well. Works like a champ. If I put it on external power and remove the USB connection the sketch won't run and the active light on the Ping unit won't illuminate. Is there something I'm missing in regards to powering the Ping? I have used 5, 9 and 12 volt supplies, all of which supply at least 750mA.
Disregard, I had a serialPrint() function that I commented out and voila! everything runs. It seems that if there's serial comm's in the program, the Arduino damn well better see a serial connection.