If the ? diskette flashes during boot up, and X won't boot, it means the system isn't finding a startup disk. Here's a possible fix.
Boot up with the OS 9.1 CD (hold down C while booting with the CD in the drive). That's OS 9.1, not OS X -- it is counterintuitive.
Once booted, open the Startup Disk control panel.
Pick the OS X drive or folder. (See, it recognizes OS X disks.)
Reboot.
Good luck.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20010531162119832