I just uploaded Happstack 0.5.0.3 to hackage. There are are two new patches:

 1. update to support mtl > 1.1 && 2.1.

 This means you can continue to use the old mtl. Or you can use the
fancy new mtl-2. The new mtl-2 is actually monads-fd + transformers.
Except that monads-fd is now a dummy package that imports mtl-2, and
mtl-2 is what used to be monad-fd. This upshot is that you can now use
Happstack with all those other libraries that use
transformers+monads-fd, assuming you get everything built against
mtl-2 / monads-fd. Or, if you want to stay with mtl-1, you can do that
too..

 The same changes have been made to the development version of darcs as well.

 See this thread for more information on why this change was made now:

http://www.haskell.org/pipermail/libraries/2010-November/014825.html

Among other things, this makes it easier to use heist with happstack.

 2. prefix unknown response codes with a space, per RFC                                                

This patch was applied by request. It fixes a bug, and shouldn't hurt
anything or break anything.

- jeremy