Is it possible for Zimbra+Exchange to Share Addressbook, while in same Email domain?

[shinacalypse]

Hi.
I have successfully tested zimbra and exchange 2003 to share the same email domain/address space for my network. When zimbra servers cant find a user's email address it forwards the email to the exchange 2003 servers, and vice versa.

My last remaining challenge is to have exchange users be able to find email accounts migrated to the zimbra servers. Zimbra already shares a GAL with the exchange 2003 servers, so that means that any user with the zimbra email client can see all the email addresses in both zimbra and exchange.
The exchange users' can't see the addresses that are not on their email server.

Is there any proven method to get exchange users (making use of Outlook and OWA) see the accounts that the zimbra servers are hosting, in the users address book?
Is there any commercial or open source tool to help make this happen, if necessary?

hope to hear from you soon, and thanks in advance.