My version of xmonad (0.8) does not seem to play very well with the newer AWT implementation XToolkit. When I tried running my gui in Eclipse, all I got was a big fat grey screen.
Luckily, all you need to do to get things working (at least in my case) was to add the following line to your run configuration:
This makes your application use the older MToolkit instead of the newer XToolkit. Just thought I should share this.