What kind of LDAP server are you using? Have you decided?
I'd recommend OpenLDAP with a mysql backend. Then you can access the
same database with your rails app, but serve the data over LDAP as well.
Maybe there is a better way, but I'm not sure there is.
Mirroring the data from LDAP could be done via cron, but that seems
messy. 
Using OpenLDAP works for us. Google it.
~k
