I'm running an Early-2008 Mac Pro with 14 GB RAM and OS 10.8.2 (Mountain Lion).
I've got Aperture 3.4.3. An hour or two ago, I dragged the App to the trash and reinstalled from my original Disk, then updated using the Apples files online, back to 3.4.3 (which didn't help).
I update Aperture whenever an update is offered, so if I remember, that was a couple weeks ago. Aperture and my library were running normally as recently as yesterday. (I keep the library on separate internal 3TB HD in one of the internal bays in my Mac Pro).
The problem arose this afternoon when I launched Aperture. It threw up a dialog telling me that I needed to use iPhoto to update the file.
Finally, I don't own iPhoto, and there's no copy of it on my HD. I used to have a copy, but I couldn't tell you how long ago that was. It was probably months ago if not longer.