I would love to see this implemented. Also, how about a way to import the Global Address Book (or list it under the Address Book Tab as a list like "Emailed Contacts"). In my opinion, that would be a great feature as well.
You could export your accounts to a csv then import & share, or in the webclient search your entire gal and drag to a address book to share etc...but I wouldn't recommend this for a large setup. It it would also then be a static not a dynamic GAL - so like the first poster stated you could make a portal to browse ldap.
You probably want to vote for: Bug 14531 - GAL via contacts folders with sorting, browsing and real sync
I'm using an external LDAP for my authentication, and I'm storing contact info etc in there. I could potentially use Zimbra's LDAP for extended directory info, but then I would have to give more people access to the admin interface, or do it myself. With my own LDAP server, I can also store whatever information I want.
No offense to the Zimbra crew, but I'd rather not have my mailserver also be my authentication mechanism for everything. (well mostly everything) Not that I think the Zimbra plugins for Samba/POSIX logins are bad or anything, just putting all my eggs in one basket is bad, especially when I have to do upgrades to the mailserver every couple of months.