"dimmreaper" (aka Jeff) turned me on to this one....
If you look at the back of the 3 pin plug, there are three slots where the metal connectors are. Take a small screwdriver, and push it gently into the slots where the black (and then red) wire is, and pull the wire out of the plug. You are pushing down a small retaining tab on the metal connector...it doesn't need much of a push to go. Leave the blue (or yellow, depends on your fan) in the plug.
You should now have two loose wires (red and black), and the third wire (blue or yellow) still in the plug. Take an open 4 pin plug coming out of your PSU. You are going to insert the loose wires from the fan into the ends of the connectors in the computer's PSU plug. Insert the RED wire from the fan into the YELLOW wire from the PSU plug, and the BLACK wire into either of the BLACK wires in the PSU plug. (I left a space between the two, but either will do)
Plug the third blue/yellow wire (still attached to the fan plug) into the motherboard fan socket..
There....your fan is now running off of the 12v lead from the PSU, and you have monitoring capibilities, too.
Thanks again, Jeff....this is a great tip! Happy to pass it along......