Cabal has a darcs repository on darcs.haskell.org.
Note: the cabal-install package now lives in the same repository as the Cabal library package (older versions had separate repos).
darcs get http://darcs.haskell.org/cabal/
darcs get http://darcs.haskell.org/cabal-branches/cabal-1.10/
darcs get http://code.haskell.org/hackage-scripts/
darcs get http://code.haskell.org/hackage-server/
To contribute patches you can "darcs record" and
"darcs send".
By default the patches get sent to the cabal-devel mailing list.
If you have access to darcs.haskell.org, and are in the darcs group, then you can push to the repository.
There are also some branches for older versions and experimental features: http://darcs.haskell.org/cabal-branches/