The Transparency tweak from Cydia allows you to change your app icons by adding a new control panel in the Settings app. As you can see from the example below, the tweak's primary function is to turn your icons invisible. There is also a shadow option and a way to turn off the icon labels. I guess this will be a great tweak if they ever invent that transparent iPhone, otherwise this is just a fun and simple way for people to mess around with their icons.
Transparency is free from Cydia’s BigBoss repo. The tweak will automatically turn your icons invisible after it's downloaded and installed. Just open your Settings app and locate the Transparency panel to play around with the different looks, or to turn your icons back to normal.