[Plugin: WordPress SEO] This plugin works with wordpress MU ?

Sort of. Joost has said in these forums that the current version is not designed to be multisite compatible but that there will be a future version that is.

That said, I’m running it on multisite and although I’m running into some issues, those issues seem to be because it’s beta rather than specifically because of multisite–ie. people on single installations are having the same issues.