I just pushed ffi-1.0.4.

This re-instates 1.8.7 support (not sure about earlier ruby versions).  Both blocking functions and non-ruby-thread callbacks should now behave similar to 1.9, albiet slower.  This means gems like ffi-zmq and curl-ffi are somewhat likely to work properly on 1.8.x now.  There is also a possibility that they will crash ruby and make your computer catch on fire, so be warned.   Only supported on posix-thread systems - win32 users will have to upgrade to ruby 1.9.2.