I've been powering my Arduino projects, lately, with a slightly modified ATX power supply.. Now, I've noticed the I can run +5V projects off the +5V from the supply, without needing to wire anything to the Vin or the coaxial connector.. I can't find the schematic for the Ethernet/SD shield (the R3 PDF here doesn't show any traces, just a almost Xray view of the board). but is the Ethernet chip powered with 3.3V? Would I be messing with fire (and a possibly fried chip) if I ran a wire from the 3.3V supply post, to the 3.3V pin on the shield? (in other words, putting power to both the 5V & 3.3V pins).. Reason why I'm asking, as many have found, the board gets super hot at the regulator, and I want to supplement the power with the supply, so it won't be as overloaded.
Am I playing with fire?
Probably, but not for the reason you think.
The ethernet shield has its own 3.3v regulator onboard. It uses the 5v bus on the Arduino to get its power. So far, so good.
Now the warning. This is from the Uno datasheet.
5V.This pin outputs a regulated 5V from the regulator on the board. The board can be supplied with power either from the DC power jack (7 - 12V), the USB connector (5V), or the VIN pin of the board (7-12V). Supplying voltage via the 5V or 3.3V pins bypasses the regulator, and can damage your board. We don't advise it.
The Mega 2560 has the same warning.
On the Mega, this has been known to brick the usb port if you supply power to the usb port and the 5v pin concurrently. It has to do with the FET that shuts off the usb power to the 5v bus when Vin > 6.6v. But if you power the Arduino at the 5v pin, the FET keeps the two power supplies connected. Bad news. Here is the topic on that.
It is quite long, but you should read reply #32 on page 3.