Ideally we'd like to get it out of beta in the 6.0 GNR series (so people feel more comfortable doing version and platform upgrades on it before 7.0 Helix arrives.) On the other hand it could possibly be just 'release candidate' next round. This is dependent on available resources for thorough vetting - we'll know more as 6.0.9 approaches.
Right now QA isn't checking Ubuntu10_64 builds, hence the beta label (when this happens you'll see the bug go 'verified'). To this point the community has only found a few minor issues (like requiring extra libltdl dependencies that we really didn't need, logrotation, misc perl, etc).
For those interested in testing:
http://www.zimbra.com/downloads/ne-downloads-betas.html
http://www.zimbra.com/downloads/os-downloads-betas.html
The more people downloading and providing feedback, both after successful installs (which we love to hear about!) along with notifying us if they should discover any possible platform specific bugs, so we can get them fixed promptly - the faster it will go GA.