A number of BizTalk users have been wondering lately that they are not able to create an x64 host on a Standard edition.
The comparison of the features of available BizTalk editions is not really clarifying this issue: http://www.microsoft.com/biztalk/en/us/editions.aspx
You find clearer information here:
where it says: "BizTalk Server 2009 Enterprise, Developer, Branch, and Evaluation Editions support native 64-bit execution on Windows Server 2008, Windows Vista, Windows Server 2003 x64 (including R2), and Windows XP Professional x64"
(The same is applicable for BizTalk 2006 R2).
The Standard edition is toned down in several areas. If you consider purchasing a BizTalk license, you should carefully define your requirements and check the available editions for the one
which meets your requirements.
Update: This is now well documented in KB 926628