Hi
You could still have a cabal-install binary in the Windows installer and not include the Cabal-1.4.x library that you built it against.
That sounds easiest, and should give all the cabal-install benefits to Windows users.
Alternatively it'd be possible to include Cabal-1.4.x too and have it not exposed by default. That'd enable cabal-install to use it for any packages that use a custom Setup.hs without it being picked up by default by ghc --make or ghci.
Unless the API 100% stable, I'd be wary of supporting a not quite finished release on Cabal in something as major as GHC. Upgrading Cabal is fairly easy, if people want to do it themselves. If Cabal supported darcs links, you could even imagine upgrading cabal with "cabal install cabal --from-darcs" - that would be wonderful! Thanks Neil