you can choose what kind of installation you want when you upgrade to leopard. you can do an "archive and install" which means that all of your data is kept and leopard is installed "underneath" and you can re-apply your data and settings from tiger. or you can do an "erase and install" which does just what it says. i plan to do an erase and install (after backing up all of my data on an external hard drive, of course) since my macbook has been acting wonky lately (actually, for a while now) and repairing permissions, resetting the PRAM, and clearing caches via the terminal has not had any effect...
