KB out yesterday (KB.921181) covers the introduction and details about the new “File Share Witness” quorum model. This is an improvement to the original MNS design that allows a read/write file share to serve as the “voter” for quorum status, eliminating the need for a third (or, at least, “odd-number”) node as voter. This new feature helps pave the way to more simplified and effective MNS clusters for use by the Cluster Continuous Replication feature of Exchange 2007 (pre-release documentation, but this section looks pretty complete).

This feature is released as a public hotfix (linked from the KB) to either Windows 2003 SP1 or Windows 2003 R2.