I remember it took me an age to work out how to permanently change my PowerBook's hostname under Tiger—if I recall correctly, it ended up requiring Netinfo Manager. Leopard killed Netinfo Manager, so it took me another age to work out how to permanently change my MacBook Pro's hostname under Leopard. For the record, and I'm not making this up, you need to open System Preferences, and change Internet & Network > Sharing > Computer Name. Sure, makes sense when you've found it, but why not just dump /bin/hostname? It doesn't work.