That's true. Bundling MacRuby with your application is easy though, and doesn't take up us much space as you might think. My MacRuby app, together with the MacRuby framework and a couple of embedded gems, takes just over 10MB GZip'd for download. Obviously that's heavier than it would be without the embedded framework, but I don't think it will be a deal killer.
Even if Apple does switch MacRuby to a public Framework in Lion, my guess is that MacRuby will evolve quickly, and many developers will choose to embed MacRuby anyway to be assured of the latest version.
