I participate in writing the Wikibook about the Haskell programming language.
GeSHi doesn't support Haskell yet, but it might in the near future (see the feature request - http://sourceforge.net/tracker/index.php?func=detail&aid=1720310&group_id=114997&atid=670234 ).
In the meantime, if i try to write <source lang="haskell">SOME CODE</source>, it is rendered as a Big Red Error that doesn't show any code. I think that it would be better to display the source in the default (black) color with a small warning saying that the language is not supported. This way, when support for that language is added, the warning will go away and the coloring will just start working.