The problem is: you can't do it straight from disk utility. I think you have to boot up from the disks if you want to partition. see:
Ok. well then I must have been confused and remembered incorrectly.
Also, how do I mark a partition as the main boot partition, so it boots that partition as default, always, instead of going to startup disk and manually selecting the startup disk each time?
Once you have the drive partitioned, you would just go into system preferences -> Startup Disk, and then select the disk in the list that you want to boot from.
So, I'm sorry that my information was wrong regarding partitioning the HDD.
But, if you can download Leopard from Apple as a developer and burn that to a disc, then perhaps you can use the disc utility included with that and then partition the disc that way.