the SDK states that
In order to build for x64 with Visual Studio 2008, you must have the "Pro" version. - The free "Express" version will not work. Instructions on using the "Express" version of newer renditions of Visual Studio are forthcoming.
I was wondering whether there was an estimated date when these instructions would forthcome. Also, I'm wondering whether it would be possible to create x64 externals with older versions of Windows and VS? I'm building my externals for Windows on an emulated Windows XP SP3 partition with VS Express 2010 and since I'm only using that partition to build my (free) library, I don't really want to invest on a newer version of the OS and/or a paid version of Visual Studio.