If you ever, for some reason, have to boot into OS 9, simply hold the D key at startup right after the chime. If you reboot, you'll have to hold D again otherwise you'll end up in OS X.
For your information, on startup, the D key tells the machine to boot on the first internal bootable partition or disk in the computer, whatever the startup disk is (useful if you're stuck on a CD). In this case, since OS 9 is installed there, it boots OS 9.
The only thing is that this trick may not be that useful in the future when OS 9 won't be bootable (not that I mind...).

