Ok, I am still learning Haskell, but I thought I would try to fix the above so that there is only one filter pass:

    qsort :: Ord a => [a] -> [a]
    qsort []     = []
    qsort [x]    = [x]
    qsort (p:xs) = qsort lesser ++ [p] ++ qsort greater
        where
            (lesser, greater) = foldl split ([],[]) xs 
        where 
            split (left, right) x = if x<p then (left:x, right), else (left, right:x)

Does this work?