there is a page that has been started on the playground yesterday:
http://www.arduino.cc/playground/Main/ShieldPinUsage
It could be better but it is a start. You will need a playground logon to modify it.
DuinoADay has a list of shields. Some of them have which pins are used listed, but not all of them.
The idea was to have the pins all listed on there but it is not a small task and I do not own all the shields and some do not tell you on the websites...
I think that linked image looked very good. Easier to see and compare shield pins. I will have a play around and see what I can do.
Mowcius