Eigen 3.1.0-rc1 has been released on June 14, 2012.
This release candidate should be last milestone before we release Eigen 3.1 next week.
Since 3.1.0-beta1, the major change is the introduction of the Eigen::initParallel() function which must be called by multi-threaded user code to properly initialize Eigen and thus avoid the risk of a race condition.