The 64 bit version is only available in Office 2010.
The reason I am asking is (quote from PC World mag article - one of many comments on this):
None of your old Office add-ins will work in the 64-bit world. They have to be rewritten and recompiled specifically for 64-bit Office 2010. In some cases, programming controls commonly used in the 32-bit world aren't even available in 64-bit. (Details are available in the MSDN Library article on compatibility.) Microsoft now offers conditional compilation support that differentiates between 32-bit and 64-bit, so sufficiently savvy programmers can write 32-bit and 64-bit ActiveX controls at the same time, but every single line of old code has to be examined and, in many cases, rewritten
Haiden W. Turner
Winged Disc Technologies, LLC
Dallas, Texas