Then you try to configure BizTalk 2006 R2 in cluster environment, you can get error similar to:
The SQL Server 'SERVERNAME' cannot be used with the SSO Administrator account 'SSO Administrators'. Local accounts cannot be used with clustered SQL Servers. (SSO)
To fix this (which worked for me) add you domain to user groups like "DOMAIN\SSO Administrators" and "DOMAIN\SSO Affiliate Administrators". Of course you also must use domain user which belongs to both this groups. And here comes nasty trick, you need to deselect and select again "Enable Enterprise Single Sign-On on this computer" checkbox, only then error goes away.