I have seen this also. i think iPhoto does this to improve the performance of the scrolling. If you have ever used iPhoto on an older machine, you will see that the scrolling can be very slow.
I assume the maximum speed of scrolling is directly related to the amount of detail you are attempting to scroll. By making the images a lower resolution during the scroll, iPhoto gets faster scroll performance. When the scroll stops, iPhoto draws in the higher resolution images.
On my MBP, this is related to the number of thumbnails that show in a window. If I make the thumbnails smaller (thus increasing the number of images in the window), the "blur" shows up.
Making the thumbnails larger (and decreasing the number of images in the window), make the blur go away.
I don't have anything to prove this, but that's my guess for what is going on here...