Hi David,
Great to here that you're going to hack on cabal. We need all the contributors we can get!
The "Do the right thing automatically" section is probably the most newbie friendly.
Other than that we really need to get the bug tracker under control. This means triaging bugs and fixing those that need fixing and closing the rest. I took a stab at this a while ago but if you want something to get your feet wet, I suggest grabbing something that looks interesting from the bug tracker.
As for hacking on cabal, I suggest using sandboxes, like so:
cd cabal/cabal-install
# only once:
cabal sandbox init
cabal sandbox add-source ../Cabal
cabal install -j --only-dep
# to (re)build:
cabal build
-- Johan