Thanks for your feedback, it helped me setting up both GHC 7.0.2 and 6.12.3 in parallel, and Cabal-install with the edited configuration file made it all much easier. I only needed GHC 6.12.3 for building 7.0.2, hence I did not bother installing / building Cabal-install for 6.12.3.

I did document the entire process here:

[k]