I am curious if it even makes consider BizTalk for the implementation of a pub/sub messaging architecture (basically what you can do with NServiceBus or MassTransit is all I rea
It's important to note that BizTalk is a server product for Enterprise Application Integration (EAI -- as Andreas mentioned). It's more involved and complicated than a framework.
Microsoft does have the Enterprise Service Bus Toolkit available to use within BizTalk, so that you can call your BizTalk environment your ESB. What they consider "ESB" may not be what you consider ESB. You can take a look at their ESB Toolkit page (http://msdn.microsoft.com/en-us/biztalk/dd876606.aspx), but it includes things like:
Of course, the publish-subscribe pattern is not the same thing as using a service bus.
BizTalk does do pub-sub well, whether or not you use the ESB Toolkit. It's extremely simple to publish a single message to the BizTalk "Message Box", and have the message routed to any and all subscribers. The pub-sub solution means that BizTalk acts as a broker, but this helps guarantee that messages do not get missed, and all messages are tracked. A BizTalk pub-sub solution has built-in extensibility points that allow us to add, change, or remove endpoints without impacting the rest of the solution.
All that being said, your requirements may not dictate extensive message reliability, monitoring, and tracking, so it could be that BizTalk is not the best fit for you. It is a large investment, and since the product can do so many different things all at once, it can be daunting at a first glance.
A new book was just published call Applied Architecture Patterns on the Microsoft Platform, which covers much of this. One of that book's authors, Richard Seroter, also published SOA Patterns with BIzTalk Server 2009, which would be essential reading if you do decide to go with BizTalk for your company.