Normally, the installer will overwrite symbolic links (for instance, if you've stored your applications elsewhere but created an Applications symbolic link at the top of your hard drive). In 10.2, I've heard that it's possible to force the installer to follow symlinks with:
% defaults write com.apple.installer FollowLinks -boolean true[Editor's note: Can anyone confirm that installers will follow symbolic links when this default is set? At this point, I'd have to call this an unconfirmed hint, as I have no idea if it works or not...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20021210060204330