This is one of the great things with the photoshop integration.
In aperture elect the photo you want to edit in PS
Right click image "edit with adobe photoshop CS5"
(aperture duplicates the original image and automatically opens the image in PS)
Edit your photo and the just click save or press cmd+s, the image in saved directly into your aperture library.
So easy....
There are also lots of useful functions for editing photos in batches and watermarking etc etc.