There are people/customers who are talking about a BizTalk Server cluster, but they actually mean a BizTalk Server group (when they use BizTalk Server 2006 R2 or older). With SQL Server you can cluster.
And new within BizTalk Server 2009: you can make also a cluster: “By taking advantage of Windows Server 2008 clustering, BizTalk Server is now able to be deployed in multi-site cluster scenarios, where cluster nodes could reside on separate IP subnets and avoid complicated VLANs.” (see: http://www.microsoft.com/biztalk/en/us/roadmap.aspx)
