A corresponding patch just went into xmonad-contrib.

Thu Mar 15 20:01:06 EDT 2012  Daniel Wagner <daniel@wagner-home.com>
 * bump X11 dependency

 Strictly speaking, it probably isn't necessary to depend on a newer X11, since
 xmonad still builds just fine with X11-1.5. However, xmonad-contrib does *not*
 build fine with X11-1.5, and we would like to avoid the confusing situation
 where someone successfully builds xmonad (with X11-1.5), then finds that
 building xmonad-contrib requires rebuilding xmonad from scratch (with a newer
 X11).

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