Hi Michael,
mojoPortal implements the MembershipProvider but is not limited to its API. We implement this API to make it theoretically possible for developers to integrate other apps by making the other app use the mojoMembershipProvider but its not possible for mojoPortal to use some other MembershipProvider because internally we use our own business objects like SiteUser which keeps our internal api flexible and extendable.
In fact, the default MembershipProvider shipped by Microsoft is the SqlMembershipProvider, which internally talks to MS SQL. By contrast the mojoMembershipProvider does not talk directly to any db it uses SiteUser and siteSettings business classes plus some internal utility logic for encrypting/decrypting passwords. So our provider can support any dbs supported by our business objects.
The only places we actually use the MembershhipAPI internally is by using the Login control, CreateUserWizard, and PasswordRecovery controls which bind directly to MembershipProvider.
Hope it helps,
Joe