"I have a new i7 iMac with a 2TB HDD and was wondering what people thought about partitioning the HDD and keeping just the OS on one partition and all of my files (iTunes, Aperture libraries, etc.) on a separate partition. Is this necessary or will there be a noticeable performance difference in anyone's opinion?"
Back in the OS 9 days, I used to maintain a partition for the system only, along with another partition for data and apps, another for "scratch" (non-essential) storage, etc.
I still do that, but with OS X, it's probably better to keep your system and apps _together_. Makes updating apps easier.
However, I still maintain my _data files_ on a separate and smaller partition. This makes it very easy to back them up, and they reside on a volume where "everything is visible" (i.e., no invisible files/folders, etc.). My home folder is in its normal place, but I don't use the sub-folders inside it for much in the way of data storage.
Another reason I like partitioning is that even for the System partition, it "keeps things compact", so to speak. During the normal course of operation, files won't get scattered all around the drive.
Partitions also make it easy to do maintenance and defrag if you're inclined to such things (I am).
One thing you might consider with so much drive space is a "backup boot" partition. Not overly large, but large enough to hold the system, some apps, etc. You pretty much just create this, and then "let it be" like a sleeping dog. If the day comes when you find you suddenly can't boot from your "main" system partition, you _might_ be able to get up and running again by booting from the backup partition. And afterwards, "attack" the main partition with diagnostic and repair software and correct the problems....