I'm fairly certain the shell32.dll file is replaced in SP1, so if you make changes now they will probably be replaced when you install it. If you already have SP1 installed now, and you plan on ...

This is an undesirable program. This file has been identified as a program that is undesirable to have running on your computer. This consists of programs that are misleading, harmful, or undesirable.

Get an icon theming program like IconPackager. Shell32 may contain a lot of icons, but very few programs are hard coded to use them. The registry has icon paths for file types, system icons, etc.