Crysis and Warhead share the same base engine, but the engine has been modified for warhead and the base code isn't interchangeable. Basically what it amounts to is we couldn't make the mod work on Warhead, even if we wanted to. When they release the SDK, it may be possible to port, but it's alot of work, so we'll have to see if it's worth the effort at that time.
But to answer your question clearly, Warhead will not work. You must have the original Crysis, and it must be patched at least to 1.2 but we're only certifying the mod on 1.2.1, so patch that high to be sure. Does that clear things up?