http://blogs.msdn.com/rich_crane/archive/2004/05/12/130693.aspxUsing enableViewStateMac requires that successive requests be forwarded to the same server (i.e. server affinity). This feature is used to help prevent tampering of a page's view state; however, it does so based on an auto-generated validation key onen-USCommunityServer 2.1 SP1 (Build: 61025.2)