This one covers BizTalk Scale-Out Configurations, from simply adding servers to your BizTalk group to fully clustering your MessageBox database on a SQL cluster. Scaling out BizTalk is a good idea for any solution which requires high availability.
One of the scenarios displays an active/passive BizTalk clustered host, which is typically only necessary if you have an adapter which doesn't run well in a group (i.e. FTP adapter -- no file locking in FTP). Most of the time I'd argue it's better to have your BizTalk servers in the same group without clustering.
Clustering SQL as active/passive, however, is almost always a good idea. Active/active I'm not such a fan of, because in a high-utilization scenario it's difficult to know if one of the clustered services could handle the load that's usually shared between two. If you like active/active, you should consider active/active/passive, which is also outlined on this poster.
When scaling out your MessageBox database, it's important to remember that you'll be going from one to three or more. The reason is that your master MessageBox database will be used for routing and subscription only, while the other (two or more) MessageBox databases will be doing the processing.