Disk Utility in Leopard and Snow Leopard allow dynamic repartitioning, but journaling must be enabled. Disk Utility help has the steps. See +*Creating new partitions on a disk.*+
Not only is it possible, it can be done.
But you should still consider two things:
backup, and booting from DVD or another (your clone backup hard drive) rather than making changes on the drive/volume/partition you are running OS X from.
Also, use SL to do any changes if you have Leopard and hope to dual boot with 10.6.
Yes, you can.
No, you don't need Boot Camp Assistant.
Apple Support => Topics from the tab above, or Help menu in any application, or pick up a reference like highly acclaimed "Mac OS X: The Missing Manual, Snow Leopard" by Pogue (960 pgs).
As the others mentioned, it may be possible. See #3 in [Formatting, Partitioning, Verifying, and Repairing Disks|http://web.me.com/pondini/AppleTips/DU.html] (or use the link in *User Tips* at the top of this forum).