app-emulation/basiliskII-jit-1.0_pre20060501 (currently keyworded ~x86 and -ppc) works (compiles and runs, even with jit) on amd64; it should have the ~amd64 keyword added. I have tested it on a couple of amd64 systems and with several versions of MacOS (~6.0, ~7.0, ~7.5), and it works fine (to be perfectly honest, I wasn't expecting it to, but I was pleasantly surprised). The jit compiler - the most likely source of any trouble - seems to have no problems, and the emulation was flawless. No difficulty was encountered whatsoever.
~amd64 added