See http://webs.cs.berkeley.edu/tos/download.html for details on what is in the 1.1.12 release, and for download and installation info for tinyos and the tinyos subsystems.
The intention behind these monthly snapshots is to get the latest code to the community without requiring developers to maintain an up-to-date CVS tree. A CVS snapshot release is very nearly that: a snapshot of the CVS tree in a convenient-to-install package. If you'd like the bleeding edge code and don't mind the inherent risks, upgrade to the snapshot releases as they become available. If stability is important to you, you might want to stay with the fully-tested releases or at least fully-test your apps before migrating wholesale to a snapshot release.
Be sure and read any comments (below) on this story for updates.
As always, please use the SourceForge bug tracking system to report problems. It is at:
http://sourceforge.net/tracker/?group_id=28656
