You won't be able to use xmonad to manage normal OS X apps. X11 will just be another OS X application sitting in your dock that you can switch to. However, it is pretty easy to install a decent set of X11 apps. If you have MacPorts installed you can install the packages firefox-x11, rxvt-unicode, unclutter, and 'vim +gtk2' (the +gtk2 variant for PRIMARY/CLIPBOARD support) to get started.