Hmmm... well, in light of three people that think my machine supports deep sleep, I'm still going to have to disagree and say that it doesn't... no offense!
From what I know, and as much as I've researched, the first desktop computers that support deep sleep were the AGP G4s. The B/W G3s and the G4 PCI machines do not.
For what it's worth, I've got a Radeon 7000 PCI in the 66MHz slot, a Rage 128 PCI in the very next 33MHz slot and a Sonnet Tempo ATA/100 in the third PCI slot -- and the Tempo card's been flashed with version 4.0 firmware, which does support deep sleep. The Radeon's been flashed with the latest firmware as well, and there are no firmware updates for the Rage 128.
Deep sleep, as far as I understand it, is the ability to sleep the entire system -- hard drives, monitors and fans. Mine sleeps two of the three, sans the fans, just as I would expect it to.
Edit: I just found this KB Article that confirms that my machine does not support deep sleep:
http://docs.info.apple.com/article.html?artnum=106790
The info is all the way at the bottom.