I've had a tool called WPMultiSite for quite awhile now which let's you use a single installation of WordPress to server multiple domains. It's more focused on the use of multiple domain. The primary use is for consultants that create and maintain many WordPress sites. It really take the misery out of updating WordPress site. With WPMultiSite you update once and all the site are updated.

The new use of the multi site term has really muddied the waters around that this means. WordPress 3.0 might make things easier to do what I need to do but so far it just seems to be causing confusion.

Also there's the issue where some host that currently don't allow users to install WPMU will in turn not allow WordPress to be installed just because it has the same capability.

Thanks
Glenn