Jesus Rodriguez has created a super cool adapter for BizTalk Server 2006 that talks to SQL Service Broker in SQL Server 2005 so that you can use SSB as a transport for SQL to SQL communications within a broader BizTalk Server ecosystem.  His adapter is available on gotdotnet and you can read more about it on his blog,

http://weblogs.asp.net/gsusx/archive/2005/09/26/425989.aspx

Thanks, Jesus!