Exchange 2007 File Share Witness Multi-subnet clustering

If you are placing the file share witness at the same site as your active Exchange 2007 server, when you have a primary site failure Exchange will not be able to start at the DR site. This is because it will only have 1 of 3 votes as both the quorum and the active are down.

You will have to run a /fq (net start clussvc /fq) to force the quorum to start at the secondary site.

Check this one out:
http://blogs.technet.com/timmcmic/archive/2009/04/26/file-share-witness-fsw-placement-and-the-cluster-group.aspx