Hello,
I have iLife '11 and have had the same sharpness problem since iLife '08. Tried all possible "solutions" and nothing worked. I did some research a couple of years ago and found this link:
http://support.apple.com/kb/TA23135?viewlocale=en_US
In case you cannot open the link here is the most important part:
"When you view an image at less than full size, iPhoto smooths it (a technique known as anti-aliasing) to get rid of artifacts that result from scaling down the photo. The relationship between anti-aliasing and sharpening is a trade-off; although anti-aliasing does not modify the actual image file, it does counter the effect of sharpening when viewing the image at less than full size.
Because of this, if you sharpen a photo, close it, then reopen it again at a less than 100-percent view, you may get the impression that your sharpness changes were not saved. In reality they were, but since the image has been re-aliased to fit your screen, the image's true sharpness may not be apparent."
(Apple Support Article: TA23135)
The article was modified on July 28, 2011, which makes me belief that the issue is still present and prevalent, and not just isolated to a few people.
I think the problem that ces161616, LRT505, and darrenfrosaltburn are experiencing is actually occurring to everyone ( I might of course be wrong), but people editing photos at 100% would not notice such problem (usually users editing small size photos or medium size photos in very large screens). Probably many people do not notice this sharpness issue (my wife for instance) because the difference could be minimal, and you will still see that all other changes remain once you exit the edit window. But for people that like to edit their photos this is really an issue.