Alex Tarkovsky
Now right-click on the panel, select Properties, and in the General tab you'll see a combo box labeled "Orientation". First set the orientation to "Left", then "Bottom", then "Top". If your setup is like mine, that's the lucky combination that lands the panel -- properly sized -- into the gap, right where we want it. If not, try moving it around a bit more with the Orientation until it behaves.
Xmonad 0.4 is rumored to make all this silliness moot by properly handling the panel positioning automatically. Until then, you're stuck with performing this routine each time you log into Gnome.
One can also resize the panel programmatically at the end of the demanage function, eg: dpy <- asks display io $ moveResizeWindow dpy w 0 (1024 - fromIntegral h) 1280 (fromIntegral h) where h = 24::Int puts the panel in the bottom, avoiding end of the routine. -- JP, a new and happy user of XMonad :)