I'm having the same problem. It appears to be happening only with photos from my DSLR (Pansonic Lumix DMC-GH1). Photos from other iOS devices: iPad 2, iPhone 4, iPhone4S don't have this problem.
The real vex is that when I go to edit the photo and start to manipulate it, the stretching goes away but crops to landscape. When I navigate off the photo the distortion returns.
To be clear my setup is:
iPad 2 Wi-Fi + 3G, iOS 5.1, Iphoto for iOS 5.1, Photostream photos from Pansonic Lumix DMC-GH1