I'm curious about whether possible to install OS X on multiple partitions. I am already using several user accounts in one installation, putting different accounts into different installations of OS X.
This has nothing to do with Bootcamp. Simply split your hard drive into several partitions with Disk Utility, and run the OS X installer. You will be able to choose whichever partition you want to install onto, and switch between each by pressing ⌥ on startup.
However, please note that this is not compatible with Bootcamp. I.e. you won't be able to have a Windows partition, at least not with Bootcamp (you'd need a solution like rEFIt).
Regarding whether you'd need to buy OS X again or not: as per the OS X EULA:
This License does not allow the Apple Software to exist on more than one computer at a time
…which I'd take for “it may exist several times on the same computer at a time”.
I know that's not what you asked for, so I'll keep it as an aside, but I can't resist saying it: this “partitioning” habit really is a switcher thing. There's no need for that under OS X, especially if you fully encrypt each partition (which I'm curious about, by the way).
The only reason I personally have two partitions is because I am totally dependent on my machine for my daily work, and I keep another OS in case of system corruption. But I aliased my user folder on the emergency partition to the one on the main drive, as an OS without my data is as useless as no computer. This fear of “a trojan” that would kill your system is irrational, and this setup will cost you much more in terms of maintenance (rebooting, keeping two systems updated in parallel…) than the advantages it will bring.