Here's a function I use in my Nukefile to compile XIBs into NIBs:
(function compile-xibs-from-to (xibs to_path)(if (and (!= target "clobber") (!= target "clean"))(then(SH "mkdir -p #{to_path}")((filelist xibs) each:(do (xib)(set nib (xib stringByReplacingPathExtensionWith:"nib"))(SH "ibtool #{xib} --compile #{to_path}/#{( nib fileName )}")))(filelist "^#{to_path}/[^/]*.nib$"))(else nil)))(set @nib_files (compile-xibs-from-to "^resources/views/[^/]*.xib$" "build/nibs"))
It uses ibtool to compile the .xib into a .nib:
ibtool source.xib --compile target.nib