About a year ago, Daniel Ehrenberg did the initial port of Factor to 64-bit Mac OS X, however the UI did not work. This is because Factor's Cocoa binding used the Objective C 1.0 API, and when Apple introduced Objective C 2.0 in Mac OS X 10.5, they only implemented backwards compatibility for the old API on 32-bit platforms. However, just recently Joe Groff updated our Cocoa binding for the new Objective C 2.0 API, and after I fixed a few bugs in the FFI on x86-64, the Factor UI now works!
I set up one of our build machines to build Mac OS X x86-64 binaries. This brings the total number of fully supported platforms to 12 -- where "fully supported" means that we have a machine in the build farm which loads all libraries, runs unit tests and benchmarks, and uploads binaries if everything passes.
It also means the end of the road as far as OS X 10.4 support is concerned. The Cocoa bridge no longer supports Objective C 1.0. However, due to popular demand we may set up a build machine with OS X 10.4 which will build Factor binaries with the X11 UI.
If you haven't yet, go download Factor and give it a spin. It's fun!