Hi,
i want to wire the standard ethernetshield to a duemilanove because i can't stack them.
so far i just wired pin 10-13 (SPI) together and the 5v to the 5v pin and GND to GND.
so far it works, though i have a problem: whenever i connect an external power supply (should be 9v, unregulated, gives 12.8v by measure) the ethernet shield seems to heat up and giving random errors.
what am i doing wrong?
I connect the external power supply via the built-in jack on the duemilanove.
when i connect a 9v battery, the power between 5v and gnd drops sometimes fluctuates, but is on 4.95v most of the time.
when i connect the power supply it drops to 0.12V, sometimes -0.12v.
please answer if you have time, i'm really stuck here, and i need to finish this for my thesis. thank you very much 
The 5100 ethernet shied chip takes a lot of power and gets quite warm under normal operating conditions. The 9v battery may not have enough for this shield. My arduino clone apparently has a bad external power regulating chip, causing the arduino voltage to be ~7.5v when connected to a 9v battery. You will just have to do some trouble shooting to see what is going on. I've made a voltage regulator out of a 7405 chip for external powering.
The problem was I was using an "fixed voltage"-supply, which gives 9v only when being loaded with 600mA.
it worked for a while now, but i was fiddling around and now my arduino just blinks rapidly on the L led when i plug it in.