"inactive" is somewhat broken, i believe. (that's why i wouldn't recommend using "sudo port -f uninstall inactive" but a "port uninstall inactive" first to make sure the right things are uninstalled.
e.g. i had optipng 0.5.4, upgraded to 0.5.5.
so 0.5.4 is inactive, 0.5.5 is active
"port list inactive" lists optipng 0.5.5
"(sudo) port uninstall inactive" uninstalls optipng 0.5.5
also, "port list inactive" appears to display every single port there is (yes even those you don't have installed) if you don't have any inactive ports.