This is from the iPhoto '08 help file:
...When you edit a photo imported to iPhoto ’08, you benefit from the great flexibility and high photo quality of nondestructive editing.It's more than just nondestructive edits. iPhoto '08's editing tools feel like simplified versions of Aperture tools, with similar photo effects. Not only that, but iPhoto '08 image editing is almost as slow as Aperture editing.
Nondestructive editing allows you to try different adjustments and effects at will⎯there’s no danger that successive edits and saves will degrade your photo’s quality. You’re never more than one version away from your original photo. Instead of saving a new version over the previous one every time you edit a photo, iPhoto ’08 simply remembers all your changes as an edit list...
I used to like iPhoto's red eye tool. I don't like the current tool -- but it's familiar. It's a simplified version of the Aperture red eye tool.
iPhoto video export is broken. Aperture doesn't handle video at all.
I think it's fair to call iPhoto '08 "Aperture-Lite", they increasingly seem to share a significant code base. In the long run this is probably very good , but in the short term it's definitely a mixed bag. iPhoto '08 is much slower than iPhoto 6 for many tasks, and then, of course, there's the nasty video export bug.
 If we don't see an Aperture update soon, it will be clear the "long run" for Aperture is fairly short. I read that Aperture is being clobbered in the marketplace by Adobe Lightroom, and I'm perpetually amazed that Aperture still doesn't support editing date metadata.