From Ben Chan:
NanoCustomizer is the perfect utility for your Macintosh, letting you do things that normally can't be done with Snow Leopard's System Preferences. Now you can hide or show hidden files (it's amazing how many there are), change pretty much everything about the login screen (including login background and logo) as well as tint or set an image as the dock.
What's new in this version:
Update: it works now. Permissions problems and bugs should be fixed.