Haha yeah, I just couldn't pass up the irony
There is actually a plugin for Thunderbird to sync contacts with Zimbra. You could install that with your portable Thunderbird. Doesn't really help with calendars, etc.. I would agree that a portable version of Zimbra Desktop would be nice, but I'd hate to have my email on a flash drive only to get stolen.. Which leads me to another question: Is it possible to password-protect Zimbra Desktop? I know you could put it on an encrypted filesystem, but the ability to secure the data at rest would be nice to go hand-in-hand with the portable offline client.