Don't recommend you run Ubuntu Desktop on your server, if that's what you're suggesting. The mail server will function best if you're not wasting CPU power on X-Windows or any other graphical bits. You can (of course) use Ubuntu Desktop for your client machines (I do) and they work great, especially with Thunderbird as your mail client.
Here's the server install guide:
Ubuntu 8.04 LTS Server (Hardy Heron) Install Guide - Zimbra :: Wiki