Forms based authentication in SharePoint 2010 has changed. It is now Claims based Forms Authentication, which means all the forms configuration stuff you see all over the web doesn't quite work in the same way. Even if you set the membership and role providers in your web
applications and central administration you forms auth still won't work!
In SharePoint 2010, several new WCF services will be handleing the mapping of the claims to the backend forms auth identities. Out of the box, claims based forms auth doesn't work! You gotta remove the <clear/> element from the Membership and Role provider sections in the web.config of the {SharePoint Root}\WebServices\Root.
Chris