Export only edited photos from iPhoto '11?
This is possible. You can use iPhoto Library Manager by Fat Cat Software, which  has saved my photos in many situations. It is frequently updated for newer versions of iPhoto.

Select your library in IPLM and Command-E or File > Extract Photos...



This results in an Extracted Photos folder and a Modified and Originals folder inside.



If you prefer not to use third-party software, an alternate method is available. Actually, each iPhoto library is actually a bundle that contains folders inside with its contents.  Navigate to your iPhoto library in ~/Pictures and it should show up as a file called Default with an icon with stacks of pictures.

Next, Control-click the file and select Show Package Contents.  If your iPhoto library is an older one, there will be folder shortcuts with "Modified" and "Originals."  Newer libraries remove these shortcuts, and the modified photos can be found in Previews.  Inside Previews, there will be several folders that outline date and time structures for iPhoto; you can just search for