I have the same problem. It's with editing photos with external editor. I've set up iPhoto '11 (version 9.4.2) with Photoshop Elements 10 (version 10.0) as external editor. I'm on an iMac running OS X Mountain Lion (10.8.2).
First I'll explain the problem, as I see it. Second I'll report what I've learn from my investions on the problem. But before this let's estabilish the basics:
When editing an original photo (that is a photo that is untouch/unchanged since the import into iPhoto) iPhoto will make a copy of this particular photo within the iPhoto library. The original photo is still in the library – in a folder called "Masters". The edited version of the photo – the version of the photo that one sees in iPhoto – is in a folder called "Previews".
The problem: Editing a photo with an external editor seems to be fine. Coming back to iPhoto both the thumbnail and the photo it self is updated as expected (now showing the edited version).
But as others have reported, in a following session of iPhoto (hereby I mean after iPhoto has been quit and then once again started/executed) I experience two problems; 1) the size in pixels stated by iPhoto in the Info Panel is incorrect, and 2) the display of the photo (double click on a thumbnail brings you to the display) is only showing a portion of the photo. The portion is a "zoomed in" of the lower left corner of the photo. I know that others have reported that they see a "zoomed in" portion of the upper left corner of the photo.
If a photo has been edited with an external editor it is no longer possible to edit within iPhoto. A message is given that this particular has been edited outside iPhoto or with prior versions of iPhoto.
What I've learned: The incorrect size/dimension of the same edited photo (problem #1) is different from each iPhoto session (see above what I mean by session). The size of the iPhoto window when starting a new iPhoto session – that is, the size of the iPhoto window when quitting the preceding session – is thus somehow defining the incorrect size/dimension stated for the edited photo. I've found a correlation between the incorrect size and the "zoomed in" portion displayed: The size/dimension of the "zoomed in" portion (problem #2) is equivalent to the incorrect size/dimension stated for the photo!
In the same session the problem #2 is only seen if the iPhoto window is in full screen mode or is maximized.
Others have suggested that it should be a problem with Photoshop (Elements). This is not the case. I've tried editing with a different external editor (Pixelmator) and still experience both problems. I've even substituted an edited version of the photo (in the "Preview" folder) with the original version (a dublicate from the "Masters" folder) and still see the problem. Actually iPhoto still gave the message that the photo had been edited outside (with an external editor). I therefore believe that iPhoto somehow keeps control of "originals" and "edited" versions in the database/xml-file – no matter the actual file.
In conclusion it seems to be a problem with photos that has been edited in an external editor (or that iPhoto believes has been edited in an external editor). This will (in a following session) trigger the problem #1. And if the iPhoto windows is maximized or in full screen mode one will also experience problem #2.