Um... well, our Flat-Panel iMac (700mhz, Superdrive) doesn't need to be hacked.. it already has a video-out port on the back, beside the USB ports. This provides mirroring on a 2nd monitor.
Unless, of course, you meant that you DID want spanning, which is a different animal.
The GeForce 2 MX (which is in my FP iMac) has a GPU capable of twin video output. The overview PDF for this card indicates that the card does support simultaneous connections to an ADC connector and a VGA connector.
However, some online discussions have indicated a firmware hack is (a) needed and (b) not available for the 15" FP iMacs with GeForce2 MX cards (see here).