Support is only provided for platforms that have been tested successfully with the product. This testing will exclude operating systems that are in development/beta as there is no guarantee the results will be valid on the released version.
In the case of Biztalk Server 2009, the server operating systems tested were:
The System Requirements guide lists the supported platforms in detail.
Currently there are only plans to support Windows Server 2008 R2 in post-2009 versions of BizTalk Server.
This doesn't necessarily mean that Windows Server 2008 R2 will never be supported with BizTalk Server 2009 or that they won't work together. I can only advise to watch out for future support announcements.
[[Edited November 26th, 2009]]
Announced at PDC 2009, Biztalk 2009 R2 will additionally support:
[[Edited January 18th, 2010]]
Also bear in mind that the version of Visual Studio that you plan to use: