Yahoo plans to backport features from Hadoop trunk to their 0.20 branch to support their needs, which has been deployed on all 25,000 of their nodes. They are working on adding security features to 0.20 by August to meet their requirements, as moving to 0.21 would introduce incompatibilities. Yahoo will likely skip 0.21 and move directly to 0.22, and they question if there is enough external interest to release 0.21.