Ideally a GUI-based configuration utility could be offered -- xmonad is an X11 application after all. If designed well, it could capture the flexibility of Config.hs. As for text file based configuration, the traditional UNIX option=value style is probably too limiting, so I concede that Config.hs should probably stick around for the more intrepid users.
I think a GUI to help set values, from a limited range based on their types, would be lovely. If someone is motivated enough to do this, the xmonad devs would happily support it. You could roll it up in gtk2hs in an hour or two, I'd imagine. Use the type information in the Config.hs file to determine acceptable ranges for values, and then have the gui constrain possible inputs to those ranges: strongly, statically typed config files, with a domain-specific gui. -- Don