find / -name *.icns -exec open {} \;

It is also better to change Preview preferences to make it open all files in one window (it isn’t a good idea to open thousands of windows). And here is the results for my system

MacBookPro5,4/Intel Core 2 Duo 2,53 GHz/4 GB DDR3/MacOS X 10.6.1

So, it’s taken about 45 minutes and 850 MB of memory to found 4144 icons on my system… It’s possible to work with all this icons in Preview, search in them and so on. But generating of PDF file from all these icons failed (in fact I cancelled it when Preview had “ate” 2 GB of memory ). Here is screenshot of all this beauty.

This could be useful for designers/programmers for researching/”stealing” icons.

E.g. I was surprised how the icon for “downloading in progress” file is drawn.