How can I convert a PC power supply to a Mac power supply for the
Gigabit Ethernet motherboard?

The gigabit ethernet motherboard uses 22 pins and has the layout shown
here:
http://docs.info.apple.com/article.html?artnum=95064

Switching the power cords from a standard ATX power supply to the
layout shown on the Apple website should work, except that there is a
28 volt "trickle" power that is needed on the gigabit ethernet
motherboard that standard ATX power supplies do no have.  How could
this 28 volt supply be obtained from a standard ATX PC power supply? 
What is this trickle voltage, why is it needed (for the ADC
connector?), can it be bypassed, and how can it be obtained?

Dude, all you need to do is add more DDR to the BIOS and the CMOS!
Even n00b's know that!

You will not find 28V on a standard ATX power supply.  You will not be
able to modify an ATX supply to supply 28V.

The 28V is marked trickle as it is always on.  For a computer to
control its own power, it requires a standby power.  With a standard
ATX connector this is provided by pin 9.  This pin is removed from the
Mac power supply connector.  Instead, the motherboard receieves its
standby power from the 28V line.  I assume this was done so that the
monitor could have standby power along with power control without
having to run additional lines through the connector.

It might (big might) work to find a 28V power supply that could be
used along with the standard ATX supply.  This 28V supply would be
connected to the appropriate pin along with its ground connected to
the ATX ground.  In theory, this would work, as the power-on signal is
provided by grounding the power-on pin.  It may not work if the supply
requires a draw on the 5V standby or if the Mac supply has additional
logic that is not present in the ATX supply.

I found a site discussing using an ATX supply on a NON-Gigabit board:
http://www.xlr8yourmac.com/systems/ATX_G4_AGP_conversion/G4_AGP_to_ATX_case_pg2.htm
I currently don't get DNS resolution, but the site is available in
Google's cache.

Personally, I would fix the current supply or purchase an appropriate
replacement.  You might check eBay for a cheaper source.

Bradley

I know this may sound silly, but is there any way to combine
volatages, say from two 12 volt sources and a 3 volt source?

You can combine supplies, but all are referenced to the same ground. 
For example, using 5V and 12V yields a usable 7V (12-5=7) difference. 
This is sometimes used to slow down 12V fans.  You cannot derive 17V
from a 12V and a 5V output, though.  Additionally, there may be issues
with outputs sinking current.  In the example of 7V, the 5V supply may
not regulate well if there was no other load on it because it would
have sink the current being supplied by the 12V output.  It depends on
the design of the supply.

The +12 and -12 supplies on an ATX could be used to derive a 24V
supply, but it would be low current (the lower of the two output
used).  This is the maximum that could be derived from an ATX supply.

For your application, 24V would be insufficient for any input
requiring 28V.  Attaching a lower voltage could damage the supply or
motherboard.

Bradley

how can you combine two seperate voltages into one?

I'm sorry I didn't make it clear.  You can combine suppies *in
general*, but not in this specific case in the manner that you want.

With a battery, you can stack them up to obtain higher voltages.  Two
1.5V batteries stacked on top of each other give you 3V.  The problem
with the type of power supply you are using is that all the negative
a.k.a. minus a.k.a ground a.k.a. (-) terminals of the "batteries" are
connected together.  To get 7V, you would take the 12V "battery" and
the 5V "battery" and place them next to each other.  The voltage
between the *top* of the 12V "battery" and the *top* of the 5V
"battery" would then be 7V.  If you tried to place the 5V "battery" on
top of the 12V "battery" you would short the 12V "battery" because,
like I said, all the negative terminals are connected together.

Now, a computer supply doesn't use actual batteries and there is no
way to "isolate" the individual supplies.  The means the best you can
get is 24V from the +12V and -12V.  The +/-12V supplies are both
referenced to that same ground as the rest of the voltages.  If you
connected a fan's negative terminal (the black wire, usually) to the
-12V and the the positive terminal (the red wire, usually) to the
+12V, then the fan would experience 24V.

If you tried to somehow attach the 3.3V voltage to this 24V setup, it
would be very bad.  You would probably burn yourself, the wire would
nearly instantly melt/burn off its insulation, and the power supply
may catch on fire and/or explode.  One would hope that the fuse on the
power supply would open, but injury may still occur before this
happens.  This would also not work for the motherboard because the
motherboard "knows about" ground through the several ground wires that
are connected to it.  It works with a fan because the fan only has two
connections.

If there were any easy way to accomplish what you want, don't you
think someone would have already thought of it?  There may be hacks in
the future on this motherboard, but, I assure you, they will not be as
simple as this.

Bradley