Nothing, actually! It's already in our Nightly builds, and will be appearing in Aurora some time this week. You can grab Nightly at http://nightly.mozilla.org
Can you give us some insight in to the decision making process when prioritizing feature additions then? It seems bizzare to me that it's taken since the first developer preview in Feburary of 2011 to get the feature in to merely the nightlies.
