There is a fantastic document over at Microsoft about the updated security features of IIS 6.0 vs IIS 5.0. It is a very good read and has some great information. You can find the document here (www.microsoft.com).
I admin many IIS boxes at work and frequent many IIS user lists and sites to add information about security and hardening. This document does not show you how to harden your IIS installation but it does explain the reasons to use the new IIS 6.0 server