These past days we migrated FWH 64 from VS2010 to VS2012. So far so good.
Also it seems to be backwards compatible with VS2010, just compiling with the flag /GS-
But what we found, and we want to share this reminder with all of you, is that this is the right way to call the CL.EXE for 64 bits:
"$(VCINSTALLDIR)"bin\x86_amd64\cl.exe
Notice the x86_amd64 folder. If you don't use this one, you will end with OBJs not compatible with 64 bits.