Does anyone have a good step by step tutorial on how to modify default images / create themes that is updated for the iPhone4? (not iOS4, but the iPhone 4 specifically.)

I am looking for a comprehensive guide / wiki that walks through absolutely everything you need to do and what tools are needed, with the specs and locations of and/or copies of all the built in art. (I assume it's easiest to modify a copy of the default images for certain things but I was never clear on how to do that with Apple's wacky image file format.)