BizTalk can certainly be used as an Enterprise Service Bus, the Microsoft Patterns and Practices ESB Guidance is based on BizTalk Server and .NET components (see http://www.codeplex.com/esb and http://msdn.microsoft.com/en-us/library/cc487894.aspx)
There are somegood discussions on BizTalk as an ESB over on the InfoSys blog (http://infosysblogs.com/microsoft/2006/09/biztalk_as_an_esb_2.html) and the Developer Network Journal (http://www.dnjonline.com/article.aspx?ID=dec05_biztalk).
You might also want to read the Microsoft ESB Guidance over on the Microsoft BizTalk pages (http://www.microsoft.com/biztalk/solutions/soa/esb.mspx).
Rgds, Nick.