The thing is, it can be a simple process. (I won't say foolproof, cause few things to do with PCs are) And it can also be an unholy PITA.
That's why personally, I recommend keeping at least OSX on its own hard drive.
The biggest multi-boot problems come from several OS's on partitions of the same hard drive. Each one can overwrite the boot sector of the others, and if you pick the wrong install sequence, it can cause the others not to boot. OSX really aggravates this because it's not really designed to be booting on a PC.
Windows gets along well with itself, so that's easy to install several different versions on the same hard drive (s). Each will be added to the NTbootloader of the latest version.
Linux gets along well with Windows- GRUB should automatically add Windows installs to its boot list. I find the GRUB menu bone simple to edit- just never delete an entry, as opposed to copy, paste and edit the entry, make sure your edit works, then delete the original entry. With a little patience and some trial and error, you can point it toward any valid OS install on any hard drive, and have it boot just about anything.
OSX is clearly a wildcard- it isn't really designed to be running on a PC. Linux won't automatically detect it. Windows will carelessly overwrite Darwin if given the chance, and why wouldn't it? So keep it separate, and avoid all sorts of trouble. I look at it like this- the cost of a hard drive buys you a new Mac- one that already has several other OS's installed on it.
Anyway, to answer the question- yes, you can install OSX on an existing partition. If it's a drive that has no bootable OS's installed, then that works fine as you're not overwriting some other OS's boot sector. But you're still creating a second bootable hard drive in the system, and you need a way to unify how to select which OS will boot.
IE: your default boot drive won't magically change itself to see OSX. You can select each bootable drive the quick and dirty way: Gigabyte motherboards usually have an F12 boot drive select at start up. Let's say you have Linux and Windows on one drive, OSX and storage partitions on another. By default, the PC boots to GRUB on HD1, which can also boot Windows. If you hit F12 at startup, you can switch it to temporarily boot Darwin and OSX on HD2. Not pretty, but it works.
Or you can add OSX to GRUB. (There are several ways of doing this, I use the boot_v8 method). Also, with Vista you can use something like EasyBCD to add OSX and other OS's to Vista's boot menu. (I've tried this, but didn't find it a better solution than just using GRUB).
The most simplified way of looking at it, is not to even think in terms of multiple OSs, but each single OS. What would you do to get OSX running on a system by itself? It really doesn't matter what other hard drives are attached to that system, and what's on them (OSs, or just files, or whatever). Just make OSX work as if it's the only OS. Make Windows work the same way. Make Linux work the same way. Now find a way to link them all together with a boot loader that lets you switch between them. Approaching it that way IS about as easy as it gets. Approaching it like "I'm just going to pile 3 or more OSs on top of each other and somehow hope they'll work, is a recipe for headache.
Check out the Multi Booting and Virtualisation forums over on InsanelyMac, and their Genius Bar tutorials - there's a lot of great info there on this topic.
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