Basically I want to turn on a computer power supply using my Arduino. I know that to turn it on you have to sink pin 14 on the ATX connector (PS_ON) to gnd. My idea is to connect that pin to a digital output pin on the Arduino, set to high, then when I want to turn it on, set it to low. My question is, since the PS_ON pin does carry current, will there be any problem connecting it to an OUTPUT pin set to high? From my understanding the PS_ON pin had a maximum of 5.25V in open circuit, but the information in this post might be more useful:
As the answer you linked to clearly states, PS_ON is an input. You can drive it with a 0-5V output pin just like any other 5V TTL input.
I think your confusion lies with the "5.25V" specification. This simply states that if the pin is completely open circuit (no current flowing), it can float up to as much as 5.25V.
If you drive the pin with a 5V I/O line, it will no longer be floating. In fact it will now simply sink a small amount of current (μA's worth) into the I/O pin which will pull that voltage down to match the I/O pin voltage. This is not going to cause any problems - if you think about it, the amount of current flowing from the PS_ON pin will be much less when you drive it to 5V than driving it to 0V.