In order to use newer versions of libffi on my 10.6.x system I intalled libffi with homebrew.

This only installed a 64-bit version. This works with all the Ruby VMs I've built with RVM (which end up as 64-bit) but causes the ffi ruby gem to fail when used with the Ruby supplied with 10.6.x.

Making libffi universal solves this problem. Topic branch with this commit here:
http://github.com/stepheneb/homebrew/commit/592c49dd53b1d6746e9b59ce15029d0fa56ae3f3

See: ffi issue here for more details: http://github.com/ffi/ffi/issues/#issue/51