The new iMac G5 will drive an external monitor (monitor spanning), as long as you're willing to zap the firmware with this Screen Spanning Doctor script, according to the AppleInsider forums.
Of course, the operation is not endorsed by Apple, and there's a risk of frying the machine.
I can say for sure the hack works on the eMac. I zapped my 2003 eMac -- a nerve-racking process -- but it worked well. It also reportedly fixes up iBooks and the G4 iMac.
