After three builds sans problems, I have one that refuses to work... I built a Diavolino to be powered from either the USB or the 5V adapter (from the starter pack). When I plug it in, there is no blinking light and I cannot upload code to it. I've checked the board at several points with a meter and it's getting a tad over 5V. The reset switch works (continuity), the LED is inserted the correct way, and the resistors measure correct. The only thing I noticed is that if I have the board powered and I touch a probe to the 1K resistor, then the LED will light. I see no obvious cracks in the board, bridged solder, etc, at least not to my unskilled eye...

Does anybody have a suggestion on how to further diagnose the problem?

Thanks!

Nine out of ten times, something like this will turn out to be a bad solder joint somewhere, so your first task should be to thoroughly check the solder joints on the board. Check for dry/cracked looking solder joints and try touching up areas that aren't obviously beautiful.

The most critical parts of the board are the crystal and two capacitors (black-stripe marked)-- check those especially carefully.

It's also possible (but unlikely) that the chip was not pre-programmed with the bootloader. If you happen to have an AVR ISP programmer (or are willing to use one of the other Diavolinos as one), you could try reflashing it, to see if it's responsive that way.

Eureka! I had forgotten about using an arduino/diavolino as an ISP. After using a pull-up resistor to disable the serial reset, I was able to flash a new bootloader and I now have a heartbeat. Thanks for the help!!