Not sure it's been added so in the next release off our mainline will have it. At this time we are releasing off the stable branch.
EDIT: A few people got confused here by 'Stable' v 'Mainline'. The 'Stable' branch is our 3.0 release. Same codeline that the 3.0.1 release came off of. 'Mainline' is where we are doing major feature development which will become out next major release. Look for this release towards the middle of this year. You can see a partial roadmap here:
http://www.zimbra.com/products/roadmap.html