Re: custom membership provider needed for existing user data ?
Actually, now that I think about it more, the Membership provider would never be involved in the User Profile page, that would be the ProfileProvider. I have actually already implemented a ProfileProvider but I haven't integrated it yet, the code is there but not being used in conjunction with SiteUser.
MembershipProvider is used by Login, CreateUserWizard (aka Registration), and PasswordRecovery controls and any Membership provider could be plugged in and those controsl would use them. But in actual practice I think you would want to have the member correspond to a row in the mp_Users table and this row could be automatically created for users authenticated against an alternate MembershipProvider. A site user row is already automatically created in the db when authenticating against LDAP or Windows and it would be easy to do this also for an alternate MembershipProvider.