The SMC Firmware Update addresses intermittent boot issues.
The SMC Firmware Update 1.0 is a System Management Control (SMC) firmware update for certain Intel-based computers, including iMac (Early 2006), Mac mini (Early 2006) and MacBook Pro (15-inch) computers.
The updates can be found here:
iMac (early 2006) SMC Firmware Update
MacBook Pro SMC Firmware Update
Mac mini (early 2006) SMC Firmware Update
Installation instructions can be found in Apple's Knowledge Base. Note that two of the three download pages have bad links to the Knowledge Base article at the time of this posting, so use this link until those pages are corrected.
Update: According to this forum thread, the update seems to be crippling some users' clock speeds.