Your master image is left untouched while you apply corrections...And the best part is, Aperture is using metadata to perform this magic, so you're *not* adding tons of Megabytes to your hard drive with each version.
I'm wondering why this is a point to make versus Adobe Camera Raw. All the reviews say "oh my god, nondestructive RAW editing!" but don't you also automatically get this with other raw editors like Camera Raw? I was under the impression that you can't write back to a raw file, meaning every raw editor that lets you make corrections before the conversion is a nondestructive raw editor. Camera Raw certainly stores your settings in metadata as well, this is especially obvious if you use the distributed cache option. So why do people breathlessly parrot Apple's marketing line about nondestructive editing as if Apple was the first company to do this with raw files? Or am I missing something?
(I would, however, like Adobe to add versions capability to Camera Raw, since it only lets you associate the latest metadata changes with the raw file. And as far as I know, Adobe's Version Cue doesn't work inside Camera Raw.)