I believe that we are having a similar issue.
We have Release 6.0.7_GA_2473.RHEL5_64_20100616202447 RHEL5_64 NETWORK edition.
I have our GAL set up with internal and external (the external is an OpenLDAP source), and it works without issue on the web client and on our Active Sync clients.
But on the Zimbra Desktop (both version 1 and 2, on Win, Linux, and Mac), only the internal GAL is synced.
On the Windows zdesktop.log for a new install of version 2, I see the initial sync, and I see each entry individually from the internal GAL.
I also see:2010-10-22 13:41:54,976 DEBUG [firstname.lastname@example.org]  datasource - Adding mapping for dataSource email@example.com__OFFLINE_GAL__: itemId(290), remoteId(uid=INT_UID,ou=people,dc=mydomain,dc=org)
2010-10-22 13:41:54,976 DEBUG [firstname.lastname@example.org]  offline - Offline GAL contact created: id=uid=INT)_UID,ou=people,dc=mydomain,dc=org name="First Last"
But nothing actually synced from external.
Unlike the original poster, un-installation/re-installation does not help.
This is consistent for us between both versions and all three platforms. And again, having an internal/external GAL always works fine on the web interface and Active Sync.
If the original poster opened a bug about this, I don't see it... is this a bug, or a server misconfiguration on my end? I'm happy to open a bug, if appropriate.