Hi Billy,
I haven't implemented this. Will share here my experience once done with it.
Regarding account creation in Zimbra. I spoke to Zimbra administrator and got a confirmation that it can be configured to use the LDAP (AD) user accounts.
So we are planning to setup an AD server and link Zimbra with it, so that I only need to provision to LDAP with zimbra related attributes too. 
~ Krishna.