Sorry it took so long, and thanks for your patience..
TouchDown has now undergone some initial testing and is now capable of running against your Zimbra account. However, this will work only with version 3.0.080 and above, and is currently in beta.
If you want to get the beta, please refer to this link:
Installing the latest beta - NitroDesk TouchDown | Google Groups
Once installed, you should be able to go into settings and follow the Easy Configuration wizard in the latest release to connect to your Zimbra server.
(Tested against a 5.0.x Zimbra server.)
cheers
-Nitro