Microsoft has published a whitepaper on Windows SharePoint Service v3 hosting solutions.
SharePoint v3 offers a lot of great features and functionality for hosters and this whitepaper talks about how to use these features to create a shared hosting solution. The whitepaper is built upon lessons learned from a dozen or so hosting partners conducted over the past several months. The guide includes things like ways to deploy SharePoint, how to use forms authentication with SharePoint, how to provision SharePoint sites, migration from WSS v2 to v3 and even a section about deployment issues in hosting scenarios.
Download the whitepaper: Creating Shared Hosting Solutions on Windows SharePoint Services 3.0.