Just wanted to offer some results and observations on the new support for cross compilers targeting windows x64.
I built and setup a MinGW 64bit cross compiler in Ubuntu which was a horrible experience in itself and several times I felt like giving up, however happily I was eventually able to build working x64 builds of Smartmontools.
I tested smartctl and smartctl-nc windows x64 builds and they seem to work fine. I also tested combining the x64 smartctl-nc with 32bit gsmartcontrol-0.8.5 and they interoperate fine.
I tested most of the functions of smartctl including drive identify, short/extended self-tests, view drive health, smart attributes and retrieving temperature logs and all the features I tried appeared to work identically to the 32bit build.
I do have an observation to make though. The compile process as-is strips out relocation information in the exe. This basically makes the exe's incompatible with the ASLR feature in Windows Vista/7. This is a minor thing, but ASLR allows you to take advantage of added security features in these OS's. This article shows how ASLR can be enabled, however it does require relocation info in the exe's to work http://eleves.ec-lille.fr/~couprieg/post/Enable-DEP-and-ASLR-on-your-applications-built-with-MinGW