I've long since given up on trying to use PSE on my Mac (even though I bought PSE 9.0 from Apple). And I'm perfectly happy, as Apple wants me to, to use Aperture to edit my photos going forward. But, although Aperture works fine for newly imported files, it's no better than iPhoto insofar as my old photos are concerned. I.e. because I "upgraded" to a new Mac running Mountain Lion, hundreds of pictures that I spent untold hours scanning into iPhoto and then editing with PSE (9.0 and earlier) are now screwed up in the way everyone here is describing (i.e. they're "mega-zoomed" and can't be further edited), regardless of whether I use iPhoto or Aperture.
Clearly, there's nothing wrong with the files themselves. In thumbnail view or when I double-click to open an individual picture, I only get the lower left quadrant; but (when I'm in iPhoto) if I touch the top of my mouse and move my finger to the right the whole picture appears. And the whole of each picture appears in iPhoto on my iPhone and iPad, which I'm syncing to my Mac.
Really exasperating that Apple hasn't fixed this. I understand that they want people to prefer their products to Adobe's, and if they don't want to support Photoshop going forward that's their perogative. But this is affecting people who relied on Photoshop when it did work well with iPhoto and that's not ok.