There are several darcs repositories related to GHC:

http://darcs.haskell.org/ghc	The main GHC repository
http://darcs.haskell.org/libraries	The libraries
http://darcs.haskell.org/testsuite	The test suite
http://darcs.haskell.org/nofib	The benchmark suite

Additionally, we have the following branches:

http://darcs.haskell.org/ghc.ghc-6.4	6.4 branch of GHC
http://darcs.haskell.org/libraries.ghc-6.4	6.4 branch of the libraries

A source tree consists of the GHC repository, with libraries as a sub-directory:

  $ darcs get --partial http://darcs.haskell.org/ghc
  $ cd ghc
  $ darcs get --partial http://darcs.haskell.org/libraries

NOTE: you really want --partial, at least when grabbing GHC. There are some 13000 patches in the repository, which take a long time to download without --partial.

Optionally, you might want to grab the testsuite and benchmark suite too, which should also be sub-directories of ghc:

  $ darcs get --partial http://darcs.haskell.org/nofib
  $ darcs get --partial http://darcs.haskell.org/testsuite