The current implementation for dialog detection only treats transient windows of the parent window as dialogs, and ignores EWMH compliant applications that set the window type to _NET_WM_WINDOW_TYPE_DIALOG

The following code detects EMWH dialog windows:

import XMonad
import Graphics.X11.Xlib.Extras

-- | Float all dialog windows
manageDialogs :: ManageHook
manageDialogs = checkDialog --> doFloat

-- | Check if window is DIALOG window
checkDialog :: Query Bool
checkDialog = ask >>= \w -> liftX $ do
                a <- getAtom "_NET_WM_WINDOW_TYPE"
                dialog <- getAtom "_NET_WM_WINDOW_TYPE_DIALOG"
                mbr <- getProp a w
                case mbr of
                  Just [r] -> return $ elem (fromIntegral r) [dialog]
                  _ -> return False

-- | Helper to read a property
getProp :: Atom -> Window -> X (Maybe [CLong])
getProp a w = withDisplay $ \dpy -> io $ getWindowProperty32 dpy a w

It's then just a matter of adding the manageDialogs hook to your manageHooks, here is my manageHooks:

myManageHook :: ManageHook
myManageHook = composeAll
               [ manageDocks
               , manageDialogs
               , manageHook defaultConfig
               ]