Boot the iMac while holding down the Option key to get to the boot menu, where the start up drives are displayed. Press and hold down the Eject key on the keyboard, and the stuck disc should eject. This tip worked great for me; I hope it's helpful to others.
[robg adds: The topic of stuck media has been quite popular here in the past. In an effort to turn today's tip into a one-stop reference for all the potential solutions, here are the methods covered in past hints:
- Power up the machine and let it sit [hint]
- Use Open Firmware [hint]
- Use FireWire Target Disk Mode [hint]
- Use disktool and System Profiler [hint]
- Grab it with the shutter from a floppy disc [hint]
- Put your Mac in a cool location for a few minutes [hint]
- Sleep then wake the Mac [hint]
- Flip the machine upside down [hint]
- Zap the PRAM [hint]

