Uh...guys.
Where the heck have you been living the last year? lbodnar, myself, and several others have been overclocking iMacs and eMacs with great results, especially the 800mHz late revision eMacs.
http://www.numbski.net/journal/imac_hack
I've been working on the 800mHz iMac, and I'm quite possibly looking to sell that and go to the new G5 iMac, and even started looking at overclocking that.
In a nutshell, what you need in order to overclock the cpu is to identify the clock chip. Once you've identified it, find a pdf doc on it if available, or call the manufacturer and get them to send you a spec sheet.
Once you have that, you need to identify the pins that control the FSB (Front-Side Bus speed) and the multipliers.
For example, by default my iMac run at 800mhz, which is a setting of 100mHz FSB, with the multiplier set at 8x. However, since from Apple my system came with RAM that ran at 133mHz, it was a pretty safe bet that the system was capable of running at that speed as well.
Anywho, if you look at my site, you'll see that lbodnar has already identified what looks like the resistor bank for controlling the FSB and Multiplier on the G5 iMac from the marketing images. The actual clock chip is obscured, so we can only speculate as to what the settings really are.