I have a 450 + DV iMac which I believe has a crashed PMU. I pressed the PMU more than once, not knowing better, then read an article on the subject. The battery went dead in a couple days, which is one of the symptoms discussed. I have read that crashing the PMU means replacing the mother board, but am not sure this is correct. Anyone know about this?
Rick 
You might be able to reset the PMU using a keystroke command at startup (kind of like ressetting the PRAM with Apple-Option-P-R).
The procedure is different for different Mac models -- my white iBook is Shift-Control-Option-Command-Power; my wife's old Blueberry iBook had a button to hold down with a paper clip while pressing Power; I'm sure my (don't call it a Lombard) PowerBook G3 has yet another variation.
Try checking the Knowledge Base at apple.com or a Google search -- search for something like "reset pmu imac dv"
What exactly are the symptoms and what were you try to fix when you pressed the PMU button? If you tkae out the battery and leave it unplugged, it should clear up on it's own, as the PMU won't have any power and should start from a clean state.
The symtoms, at this point, is only the battery dying in the amount of time the service manual said it would if the PMU is crashed. It isn't a new battery, and may have just died. Too much of a coincident for me to believe it. I plan on replacing the battery, and resetting the PMU (properly).
As to what I was doing; I bought the iMac on ebay from a junk dealer for $60 shipped, and found it had been protected with a Firmware Password. After reading through the KB article on the subject (which said you could remove a firmware password if you had access to the inside of the computer, but didn't explain how), I figured the only thing to do would be to reset the PMU. (which didn't work, by the way). I have since learned how to remove the password, replaced the battery with a used one, flashed the PRAM, installed Tiger as a clean install, and tested its functionality. All is well with the exception of the battery going dead.
Rick