Yeah, its not a problem with your iPhoto, its a general iPhoto problem, it doesn't happen to all photos (I find that it happens most after I edit photos).
For example, this is a photograph of a Dragonfly I shot in Greece. It has been processed and cropped in Photoshop CS and after Importing it into iPhoto, I opened it in a new window, maximized the window and set the photo to "fit to window." Notice how it looks very soft. If this is what the photo turned out like, I would be very disappointed.
This is the exact same window, the only difference is that I have set the Image size to 100%. Notice how much sharper it looks, this is how the image
should look.
I've read about this problem at a few different websites. Its not a huge problem, but for photographers its a real pain in the backside, especially when we have hundreds of images to review as quick as possible and having to fiddle with settings just to see if the image came out sharp is a real turn off. The only thing I use iPhoto for is to store all of my images post-processing, this gives me a great app to view all of my 'keepers.'