I'm having this problem, too.  I've tried everything I've seen on forums, but still no luck.  I'm trying to get it so that if you log out, it will remember the username and password and if you checked the remember me box or not.

 Form:

    <asp:Login ID="Login1" runat="server" RememberMeSet="true" CssClass="loginClass" TitleText="">
    </asp:Login>

 In the web.config:

<forms name=".ASPXFORMSAUTH" protection="All" defaultUrl="/admin/default.aspx" loginUrl="/login.aspx" cookieless="UseCookies" timeout="600000" slidingExpiration="true">

 And I also added a machineKey.  What else am I missing?  I would have thought that it would remember the username and password automatically.