From Microsoft:

Today we have made available the Microsoft ASP.NET v1.1 Member Management Component Prototype on the ASP.NET site.
You can download it at http://www.asp.net/Default.aspx?tabindex=6&tabid=41
Comments, questions, bugs should get posted on the forum at http://www.asp.net/Forums/ShowForum.aspx?tabindex=1&ForumID=186

The Microsoft ASP.NET v1.1 Membership Management Component Prototype contains classes that allow a developer to more easily authenticate users, authorize users, and store per-user property data in a user profile. The authentication feature validates and stores user credentials which a developer can use to manage user authentication on a web site. The authorization feature lets you treat groups of users as a unit by assigning users to roles such as manager, sales, member, and so on. Combined with ASP.NET's built-in authorization functionality, Windows Shared Hosting developers have end-to-end support for maintaining user-to-role mappings and authorizing users based on this information. The profile feature enables you to provide users of your Web site with a custom experience. By defining and using profile properties, you can track any custom information your application requires, including user information and user preferences.

There already two applications in beta using this component; DotNetNuke by Perpetual Motion and Community Server by Telligent Systems.

Important: The functionality provided by this component is a preliminary version of the Membership, Roles, and Profile functionality coming in ASP.NET 2.0 and will change in the final release of ASP.NET 2.0. This means that any ASP.NET v1.1 applications you develop using this component will need to be updated when you migrate to the final release of ASP.NET 2.0. This is also a non Microsoft supported component.