System Preferences > Energy Saver > Options tab > "Allow power button to sleep the computer"
This option was present on my 400Mhz G4 running 10.3.2; however, it was not present on my 1.5GHz 17" PB running 10.3.7. I can't conclusively say it's a difference between desktop and portable Macs, but I'm sure somebody could corroborate that for you.
I also have a sneaking suspicion that such an option, even if not present in System Preferences, may be accessible though the power management command-line utility whose name escapes me at the moment.