After installing the Apple Security update 2006-004 I had problems starting Mail.app and Safari.app on my iBook. The problem report I sent to Apple said:
Symbol not found: __ZN3KJS9ObjectImpC2Eb
Referenced from: /System/Library/Frameworks/WebKit.framework/Versions/A/Frameworks/
Expected in: /System/Library/Frameworks/WebKit.framework/Versions/A/Frameworks/
The rest of the system seemed fine, and I was able to use Firefox to find this simple solution (after tons of useless problem reports without solution):
Just install the MacOS X Update 10.4.7 Combo PPC update again. This solved the problem for me. Of course I don't have the security update anymore, but quite frankly, I can live with that.
Btw, I found this solution on another blog.
On days like these I really like blogs.