Nice!

I minor tweaked the code and have added it to the wiki http://wiki.github.com/ffi/ffi/windows-examples#intermediate  Review and revise as needed.

I removed the file encoding comment so as not to imply it's required, and I tweaked the function signature and invocation to be a bit closer to the MessageBox API documentation.

Probably makes sense to try 'jcode' with MRI 1.8 and see if it can be made to work.  The code doesn't work with JRuby 1.4.0 using --1.9 due to undefined "String#encode" and I haven't yet tried JRuby 1.5.

From the FFI API perspective, can the Ruby string code be de-uglied with a combination of some clever #ifdef compile-time and runtime check magic?

A new type binary blob type similar to :buffer_in but skips over the null-byte checking and encodes to UTF-16LE behind the scenes?  Something like :unicode_string or :win_wide_string?  Could the new type also be made applicable to both Windows UTF-16 and *nix UTF-8/locale environments?

Haven't thought about windows SBCS, DBCS, and MBCS C runtime code page issues, but having an explicit type to help with Unicode WinAPI's could be helpful and less error prone.

??

Jon