I'm having a similar problem.
Just purchased a used G5 2.0GHz yesterday and gave it my usual deep cleaning (all fans, chassis, etc).
When I put it back together and tried booting it up, I got the chime and then nothing...
OK, I'll try pushing the PMU button. Same results.
How about a fresh new battery! Nope, still a blank screen and no hard drive noise.
"Hmmm? Maybe I got a lemon?" I think to myself, and walk away.
After thinking about what to do next for about 15 minutes, I came back to "What's this!? A desktop!? Sweet!" Everything is working right, so I set up Date/Time so on and so forth and reboot... Chime and blank screen, but I sit and watch. After about 3 minutes, the machine starts to rev it's fans up faster and faster, then my mouse lights up and the computer starts the usual boot-up.
OK, this is weird. Then it occurs to me that when I set up my display (Dell 17" CRT using a DVI to VGA converter), the computer listed it as a generic VGA Monitor.
How about trying an Open Firmware reset? Viola! After that was done the computer now saw the monitor as the correct brand and model.
Rebooting now was happening with no lag or delay. It's fixed right?
Well, I needed to take the computer to another room to run the software updates so I unplugged it, relocated it, and reconnected it. Chime then blank screen... Come on. This is irritating.
While it was booting I tried cycling the power button on the monitor and it immediately started to boot up.
Looked at the Monitor Preference Pane and it's listed as a VGA Monitor again. Go through another Open Firmware Reset and the monitor is listed correctly with name and model and the computer boots up with no delay.
I am going to try my other primary monitor tonight (Samsung LCD with DVI) to see if this is all happening due to some glitch when the computer doesn't know what kind of monitor is hooked up. Could be something with the DVI to VGA adapter (Apple Brand mind you) or maybe its this old Dell CRT.
For fun, and to possibly save some time for people reading this, I did go through the long full hardware test and everything passed. Weird thing about that was the fans stayed at full power during the ~45 minute test. Sounded like a jet plane in my living room! Wow!
Hope this helps someone. Time for a beer now. Cheers.