In an Oct '28 post titled "Changing the game: BizTalk Server 2010 and the Road Ahead", BizTalk team sounds as though Microsoft's thinking about BizTalk vNext evolving into something like a "BizTalk Azure" offering (akin, at least in concept, to Windows Azure and SQL Azure).
"Our plans to deliver a true Integration service -- a multi-tenant, highly scalable cloud service built on AppFabric and running on Azure -- will be an important and game changing step for BizTalk Server, giving customers a way to consume integration easily without having to deploy extensive infrastructure and systems integration.[…]
However, regardless of the innovative investments we're making in the cloud, we know our BizTalk customers will want to know that these advantages can be applied on-premises (existing or new apps). We're committed to delivering this new "Integration as a Service" capability on-premises on AppFabric server-based architecture. This'll be available in the 2 year cadence that's consistent with previous major releases of BizTalk Server and other Microsoft enterprise server products."