On OS X Snow Leopard, you may have libs of different architectures installed and many binaries are universal binaries, meaning that they are cross compiled for several architectures, and the best architectures is selected at run time.

This may cause issues when you go to run your FFI code using 64bit ruby against a lib that is 32 bit for example you may get an error similar to this :

/Library/Ruby/Gems/1.8/gems/ffi-0.6/lib/ffi/library.rb:61:in 
`ffi_lib’: Could not open library ‘libmylib.dylib’: 
dlopen(libmylib.dylib, 9): no suitable image found.  Did find: 
(LoadError)  /usr/local/lib/libmylib.dylib: mach-o, but wrong architecture. Could 
not open library ‘libmylib’: dlopen(libmylib, 9): image not found   from /Library/Ruby/Gems/1.8/gems/ffi-0.6.2/lib/ffi/library.rb: 
43:in `map’  from /Library/Ruby/Gems/1.8/gems/ffi-0.6/lib/ffi/library.rb: 
43:in `ffi_lib’   from xxx.rb:14

Using lipo (man page for lipo) to examine the lib an find out what architecture it is, we can see that it is is i386)

$ lipo -info /usr/local/lib/libmylib.dylib 
Non-fat file: /usr/local/lib/libmylib.dylib is architecture: i386

Running lipo on my stock install of Snow Leopard ruby, we can see that it is fact a fat binary(universal binary)

$ lipo -detailed_info /usr/bin/ruby 
Fat header in: /usr/bin/ruby 
fat_magic 0xcafebabe 
nfat_arch 3 
architecture x86_64 
    cputype CPU_TYPE_X86_64 
    cpusubtype CPU_SUBTYPE_X86_64_ALL 
    offset 4096 
    size 14176 
    align 2^12 (4096) 
architecture i386 
    cputype CPU_TYPE_I386 
    cpusubtype CPU_SUBTYPE_I386_ALL 
    offset 20480 
    size 14112 
    align 2^12 (4096) 
architecture ppc7400 
    cputype CPU_TYPE_POWERPC 
    cpusubtype CPU_SUBTYPE_POWERPC_7400 
    offset 36864 
    size 13904 
    align 2^12 (4096)

To force ruby to use the i386 arch we can use the arch command (man page for arch)

$ arch -arch i386 /usr/bin/ruby libmylib.rb

and everything shall work correctly.