The download of the Dynamics CRM SDK includes the sdk assemblies (microsoft.crm.sdk.dll, microsoft.crm.sdktypeproxy.dll etc). There are 4 sets of these assemblies, one in the bin directory, another in bin\64bit, and two more in the respective online subdirectories.
Given the directory naming, I'd always assumed that the assemblies in the bin directory were built for 32bit (x86) systems only, and those in bin\64bit were build for 64bit systems. I found this a little annoying, as I generally prefer to build assemblies as MSIL (AnyCPU) to avoid the need for different 32 and 64 bit deployment packages.
However, it turns out that the assemblies in the bin\64bit directory are actually built as MSIL (AnyCPU), rather than specifically as 64 bit assemblies (apparently this was an earlier deployment mix-up which it's now too late to correct). This gives me what I want, so I now always use the assemblies that, bizarrely, come in the bin\64bit directory of the SDK.