ruby-ffi is somewhat broken w.r.t closures/callbacks on snow leopard,
due to the NX bit being enabled on heap allocations now, and a
mis-configuration when building the internal libffi - the code to
properly allocate closures as read+execute was not being compiled in.

To add to the fun, if you build libffi out of macports, it also does
not enable that code, and ruby-ffi will find that version and try to
use it, and fail miserably.

So if you need FFI on 10.6, install 0.5.0 from source.