Wordpress has finally released their multi-user version, WordPress MU. It is open source, like WordPress, and allows you to host stacks of blogs on the one box and manage users across them.
We are yet to consider moving to this version (despite now having 5 public blogs on our sites!) – mainly because of our current reliance on IIS/Windows – but those who have a little more flexibility with their chosen hosting platforms would be wise to consider moving to the multi-user version.