The Debian Release Team has shared that on 30 June they plan to stop the automatic migration of packages from unstable to testing, which is "freezing" in the road to releasing Debian 7.0, which is codenamed Wheezy.
While it will be frozen at the end of the month, the stable Debian 7.0 isn't expected for release until early 2013. "As with the Squeeze freeze the process will be gradual, with a more liberal acceptance policy in the earlier stages. Precise details as to what that means will be available soon."
More details at debian-devel-announce.