To anyone who has managed to 'paperweight' their Mac when trying to install OS X, there is a solution to try! In essence, your Mac uses Open Firmware to boot itself up, and this symptom (dead Mac) usually means the Open Firmware startup variables in NVRAM have been corrupted.

In order to reset them, do the following:

  1. Boot the Macintosh while holding down the Command, Option, O and F keys (Cmd-Opt-O-F)
  2. You should get a monochrome text screen with a prompt. This is Open Firmware.
  3. Type 'set-defaults' and hit return
  4. Type 'bye' and hit return Type 'reset-all' and hit return (Thanks Millennium!) this point, the Mac should reboot and start looking in its 'normal' locations for a viable Mac system folder; i.e. on the boot hard drive, and on the CD-ROM. From that point, you can either continue working or try to reinstall.

As Millennium mentions, there are other methods of resetting the NVRAM and/or entering Open Firmware if the Cmd-opt-O-F doesn't work. First and most brute-force; you can remove the small battery from the motherboard of the Mac and unplug it from wall current. Let it sit for a minimum of 10 minutes. This should drain the power maintaining the NVRAM, resetting all values to their defaults. Second, you can hold down the programmer's switch (the second hardware button, next to the 'reboot' button) on the Mac while powering on. After ten seconds, release the button; you should drop into Open Firmware.

Note that interrupting Mac OS X installs can easily cause this problem, so make SURE you do not reset the Mac during an OS install!