OK, after DiskBreaker pointed out nvram and I confirmed that the Startup Disk is saved in the non-volatile RAM (via some technote, I don't remember which), I started playing...
/usr/sbin/nvram -p will list all the nvram variables
nvram -h (or any other invalid option) will list all the valid nvram options
I piped the output of nvram -p into a file, ie
nvram -p > osXStartup
Then, I opened up the Control Panel and set the Startup Disk to my Classic partition, then I did:
nvram -p > classicStartup
checking the files for differences:
diff osXStartup classicStartup
< boot-device /pci@f2000000/pci-bridge@d/mac-io@7/ata-4@1f000/@0:10,\\:tbxi
> boot-device /pci@f2000000/pci-bridge@D/mac-io@7/ata-4@1f000/@0:9,\\:tbxi
So, it should be possiible to set the startup disk from the command-line (nvram name=value). Note that I haven't tried this and I don't know if it will work or if it will screw up anything.
I hope to setup a script to change the startup disk to Classic. Anyone have any idea how I could write a script/program to change the startup disk to OSX from Classic? The office backup software only runs on Classic, so I have to reboot into Classic, then shutdown for the backup software to work (on shutdown it waits for the backup server to back up the data, then it actually powers down). I'd like to have a script in the Classic Startup Items folder to change the startup disk to OSX so then I could just run the script in OSX to change the startup disk to classic and restart. The Classic script would then change the startup disk and wait for backup then shutdown... As it is, I have to do it all manually before leaving work every day.
(Yes, I've been avoiding using Classic for anything when at all possible...)