Not really a hint - but this Knowledge Base article lists the start-up keys of Intel Macs. Could be useful ... if only I had one!
[robg adds: This list looks identical (as of now) with that of the PowerPC-based Macs, except for the removal of the Open Firmware booting instructions. Perhaps it will be udpated in the future with additional keys, such as a way to reach the Extensible Firmware Interface (Wikipedia entry)?]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060123094219578