Did you really export it or did you set PSE as your external editor? If you used file>export in iphoto, then you must import the edited photo back to iphoto as a new file.
To avoid that you can set PSE as your external editor in the iphoto preferences, and iphoto will remember your last saved edits + original, but you must not change the name or format of the image when saving. If you see the save as window it won't work. You may need to go to the PSE preferences>saving files and set On First Save to save over existing to avoid seeing the save as window. It will not work with raw files.