2010 iMac Target Display Mode not working

I'm trying to use my 27" 2010 iMac with OS X 10.7.5 as the display for my Power Mac G4 using Target Display Mode, but command+F2 doesn't do anything. I have it hooked up from the G4 using a DVI -> Mini Displayport adapter into the iMac. I've tried having the G4 plugged into the iMac, shutting both down, booting the G4 first and then the iMac. I've tried repairing my disk permissions on the iMac. I'm using an Apple keyboard and have tried both a wireless and USB keyboard, still no luck. Is what I'm attempting impossible or am I doing something wrong?