I got stuck on the gray screen on one of my machines after install, but I was able to get past it by disconnecting all USB devices and restarting.
Other things worth trying:
- Shut down. Disconnect the monitor and start it up. Shutdown again and reconnect the monitor.
- Reset the PRAM by holding down command-option-P-R at startup.
- Reset the firmware by holding down Command-option-O-F at startup, then typing "reset-all" followed by return.
One thing I'm learning, mostly from others' experiences: Always Repair Permissions before applying a system upgrade. That said, for most users 10.2.3 has been a flawless upgrade.
PS: To restart when the system is frozen, on modern G4 machines simply hold the power button in for ten seconds.