Just one year ago I spoke with one guy who sold his iMAC (last generation G4 with 17") with a nifty change:
You could hook up an video signal so it was displayed on the screen. I believe it must have been then a digital signal.
Who knows more about it. Unfortunately I lost the email address of this person.
To put the ideas further: I just bought an iMAC G5 with 20" screen and I would like to use this machine with my G5 Dual as backup and workhorse:
1. The screen when needed as additional viewing part for the main screen of my G5
2. The computational power of the iMac via iGRID (see also my forum topic in "Software Hacks")
With this you could have everything neatly in one place (wired with Firewire over IP or Ethernet) and have a multitude of uses which a solitude screen (eg. 20" AL Cinema Display) doesnt offer. (The new Mac Mini is only G4 power and further iMACs might even have Dual G5 Power)
I think it is worth digging for it.
Just another idea: the monitor cable hooks into the motherboard with a specific clamp. This opens perhaps the door for a hardware workaround if the software hack doesnt seem to be available or feasible. You would need the electric definition of every pin of this clamp, but this shouldnt be too difficult to find out. Also a sort of "semi-auto-switch" like available for USB Bus systems would do the the switch-over between internal use for the iMAC G5 and external input from any other source.
Please let me know what you think and know. I am happy to share every further developments with you.