Are you able to open and edit a photo outside of iPhoto with CS5? When you selected CS5 as the editor in iPhoto's preferences did you select the folder that holds CS5

or did you select the application:

Some have made the mistake of selecting the folder. Be sure you haven't.
The following are tips on using Photoshop from within iPhoto:
Using Photoshop CS3 (or Photoshop Elements) as Your Editor of Choice in iPhoto.
(the PS screenshots are from CS3)
1 - select Photoshop as your editor of choice in iPhoto's Advanced Preference Section's under the "Edit photo:" menu.

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2 - double click on the thumbnail in iPhoto to open it in Photoshop. When you're finished editing click on the Save button. If you immediately get the JPEG Options window make your selection (Baseline standard seems to be the most compatible jpeg format) and click on the OK button. Your done.
3 - however, if you get the navigation window

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that indicates that PS wants to save it as a PS formatted file. You'll need to either select JPEG from the menu and save (top image) or click on the desktop in the Navigation window (bottom image) and save it to the desktop for importing as a new photo.
This method will let iPhoto know that the photo has been editied and will update the thumbnail file to reflect the edit..
NOTE: With Photoshop Elements 9 the Saving File preferences should be configured as shown:

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s I also suggest the Maximize PSD File Compatabilty be set to Always. In PSE’s General preference pane set the Color Picker to Apple as shown:

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NOTE: If you want to use both iPhoto's editing mode and PS/PSE without having to go back and forth to the Preference pane, once you've selected PS as your editor of choice, reset the Preferences back to "Open in iPhoto". That will let you either edit in iPhoto using the Edit button or Control-clicking on the thumbnail and selecting "Edit in iPhoto" or in PS/PSE by Control-clicking on the thumbnail and selecting "Edit in External Editor" in the Contextual menu.

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This way you get the best of both worlds.
OT