This week, Windows Azure Infrastructure Services (IaaS) was made generally available to the customers. You can look at some blog posts related to this announcement here and here. As part of this, BizTalk Server 2013 (RTM) VM images were also pushed live and it becomes all the more easy to set up a BizTalk Server machine on the cloud. BizTalk Server is available on IaaS in three flavors:
You can read more about each of these SKUs at http://msdn.microsoft.com/en-us/library/jj248689(v=bts.80).aspx. This link also provides information on how to set up a BizTalk Server VM running on Azure.