Problem: I have written my own membership provider to store and validate user information. Further, I have a web page that allows a user to come and sign up with a username, password, and a whole host of other client-related data. When the user submits that data, I would like to have them logged in.

Solution: Since I'm using Forms authentication for the ASP.NET website, it appears that the following code-snippet does what I want:

if (Membership.ValidateUser(u.UserName, u.Password))
    FormsAuthentication.SetAuthCookie(u.UserName, false);