simonsimon all the Mac is doing is a built in security service. It's to prevent people installing software on a Mac computer without the 'administrator' knowing it. Macs are primarily an industrial computer, used in work places or schools. Not the kind of places that would appreciate Limewire.
You need the Administrator password whenever you make a system change or install software regardless off permissions. if it worries you that much, install limewire while offline, then change the admin password before going online and using limewire.