That would be nice, give it a shot if you have time and motivation.
I think basically there would be a need to import the roles/groups from LDAP into the mojo db and a page for mapping them to mojoPortal roles.
Then in the mojoRoleProvider we would need to add some logic to lookup the user's roles/groups from LDAP and then set the role cookie by replacing those with their corresponding mojoPortal roles.
Ideally it should be platform neutral as it is now rather than Active Directory specific. We are currently using the Novell.Directory.Ldap.dll as a wrapper for LDAP operations. It comes from the Mono project and is fairly old and might should be updated to a newer version.