Arduino w/ethernet must be reset after power-up


I'm using the standard Arduino boards with two configurations:

  1. One using the Wiznet ethernet shield
  2. The other using the standard (Sparkfun) ethernet shield

I'm encountering a curious problem where both configurations do not function when first powered up. I must press the reset button on the Arduino and then everything runs correctly. I'm using a wallwart power adapter to power both Arduinos.

Any ideas?