[xmonad] Plasma hiding xmobar

[Matteo]

Hi,
I installed xmonad and xmobar. Everything goes fine, except that 
sometimes (not always) my xmobar gets covered by KDE's plasma desktop. 
I'm running Kubuntu 9.10.
It's quite annoying, since I miss the clock on the bar.

screenshot:
http://img5.imageshack.us/i/screenac.jpg/

you can see the space on top of the screen, where the bar should go. It 
usually works, but it seems random to me, any help would be greatly 
appreciated.

Thank you,
Matt.