I work in a high-EMI environment. A couple of small changes to the standard Arduinos would ease matters for those like myself:
Add in a low pass filter for the AVCC pin - a simple LC filter (15uH, 10nF) as recommended by Atmel - this will also enable more accurate ADC readings...
read AVR040 & AVR042 - it's not commonly known that the /RESET pins on most AVRs is not ESD-protected in the same way as all other I/O pins - its almost impossible to find this out in the data sheets - its very obtuse indeed. A small diode (1N4148) with a 4k7 resistor to VCC, and a 4n7 cap to ground would help greatly (assuming you're not using HV programming). In addition, leaving space for a small zener between GND & /RESET would allow it to be easily added in very high noise environments.
Probably only a few cents to add, but these are simple changes that are recommended by Atmel that would, IMHO, greatly improve the robustness of these boards.