It's easy. Set CS5 as your preferred "external photo editor" in iPhoto's preferences. Now when you choose to edit a photo in iPhoto, CS5 will open, you make your changes, save and the changes are saved in iPhoto. Simple!
For the pictures you've already edited in CS5, just save them temporarily to a folder somewhere like on the desktop, then drag those "new" photos into iPhoto. Assuming the metadata hasn't been damaged, they'll automatically be added to the right events. You can then delete the folder on the desktop. You'll have two copies of the photos (the unaltered and the altered one) but you can decide which one to keep at any time.
