I wanted to add a new fan to my 12" Powerbook. How likely would it be for me to actually use the power from the motherboard to do this? If i used a fan with a higher voltage rating, will it blow out, or will it just run at a lower speed?

It will run slower...

No problems on the motherboard then?

As long as you are using the existing fan power supply, no. If you are trying to add a second fan, you may be asking for trouble (and certainly shorter battery life).

Having said that, you might want to look at the wattages involved if possible. Lower voltage will simply mean lower speed, but if its trying to pull more power than the old fan, you could in theory burn something out. Its not an issue on desktops, as the power supplies are rated in the hundreds of watts, but laptops (12" PBs run on 45W) are much more strict on power distribution. Even if you don't put anything physically at risk due to extra current or heat, you could conceivably corrupt the power manager (every time the fan comes on at least), and that can cause all sorts of weirdness.

I suggest using a USB connection, external or internal.  That will getcha 5v and more than enough wattage.

If it has firewire then use the firewire port/s since they have more voltage.

hmm, if i can find a good fan that'll fit, i might give it a shot.
thanks for your input!