I was recently had problems using an ethernet shield with a Arduino Pro (5v/16Mhz) unless it was hooked up to the FTDI cable.
It turns out that the total current draw for both the shield/board was 190-210mA, and it was causing the voltage regulator on the Pro to trip. Once I implemented my own regulator with a 7805, it worked fine.
It seems odd that the Pro can only source 150mA, since the wiznet 5100 ethernet chip can draw that much all by itself. Have others had problems getting ethernet shields to work?
– Mitch