There are some reports that, in transferring your images from your iDevice to your computer, the down-sampling routines are a little too enthusiastic, causing your images to lose some clarity. This is happening after upgrading your OS to iOS4.
I’ve not seen this myself, but then again, I try to take photos with a camera, rather than a phone, whose lens may well have been smeared with greasy fingers while being used as a phone, media player, address book, iFart generator …