There's a Jailbreak bug that affects Springboard (the Home Screen system on iPhone and iPad) and causes your icons to go fuzzy and disappear. Depending on which version of iOS 4 you're on, and which Jailbreak tool you used (limera1n, greenpois0n, redsn0w, etc.) you may or may not experience it or be able to replicate it, but if you do we have a simple if temporary fix for you after the break.
Though the disappearing/fuzzy icon bug isn't much more than a annoyance -- you can still use the icon in question -- is also quite easy to go around.
You can tell if you have the problem simply by swiping quickly from Home Screen to Home Screen. If you you have the bug, some icons will disappear or turn fuzzy. To fix it, just swipe some more and they'll eventually re-render.
Again, this bug is not incapacitating for your iPhone but it is a bit disconcerting when you see one of your icons has disappeared. (Luckily the name remains under the icon so you can still figure out which app it is).
This bug seems to have started after iOS 4.0 came into effect. We will have to see if it is still in effect in iOS 4.3.
If you've had this problem let us know which version of iOS you're using and which tool you used to Jailand come up with any better fixes, let us know in the comments!