FreeBSD team, for example, actually removed "Giant lock" from most important drivers, make memory allocation routines SMP-aware, replace default scheduler and they have been ported their codebase to more than one 64-bit architecture. It is not just recompilation with -m64 flag. But they failed to convince people that they are doing many thing much better as a server OS, and already have all that "innovations", except may be OpenCL.