Yes, I'm seeing the same issue, just noticed it after the upgrade triggered when i opened up iPhoto this morning.
Seems to only happen to portrait images, thumbnails and main image, only fix i can see is to select the image then go to Photos > Revert to original. The portraits it re-sized seem random across various events, older events are fine some newer ones too, so not sure if it's due to photos that were imported during X version of iPhoto. Alot of the ones i see scewed are from 2010 after that I don't see any effected... odd.
The fix is a bit annoying and time consuming, I suppose I could locate the errored out events then restore from time-machine?
Let me know if you come up with anything, i could try the rebuld option I guess? I wish you could break up the libray database into year segments or something similar so it's not all or nothing... so to speak.
Jeremy, I found that 'reverting to original' solved the problem too (after locating the image in the Finder and discovering that it either was or wasn't rotated to the correct orientation), then rebuilding the thumbnails. In fact running pretty much all the rebuild options!
It's quite frustrating that one can't select the event or individual images on which to run the 'rebuild' command; I have approx 15k images in my library and even with my 2ghz i7 with 8gb RAM it takes about 2 hours.
I noticed it was the portrait images only affected as well, and only the last import (so far, not had time to check the whole library). Luckily, I had already done all my editing in Photoshop prior to importing, otherwise I would have lost it all when reverting to the original version.
I would love to play around with it a bit more but stuff like work gets in the way