Avocent are the best for this - multi user switches with centralized authentication on a per port basis.
http://www.avocent.com/web/en.nsf/Content/DSR8030 8 concurrent user / 16 port version (you can daisy chain non IP switches from the ports and retain per port access control) They support P/S2 / USB / Sun and serial - though are a very expensive way to do serial. Simon
