http://factorcode.org is now running Factor 0.80. I've also been working on the AMD64 FFI. Calling functions with integer and floating point arguments now works. Functions taking more than 7 parameters do not work, however; I haven't implemented stack parameter spilling yet. However, it is not too hard; I just have to adapt the existing PowerPC code that performs this task.
A release of 0.80 is just around the corner. As soon as the AMD64 FFI works well enough to run the Factor UI, I will release it.