I have upgraded to Cabal 1.2 while still using GHC-6.4.1. In the Haskore project there is the module NewResolutions.lhs which let GHC run into extensive swapping (certainly due to excessive memory consumption) on compilation when compiled with optimization. One can say, it cannot be compiled this way. This is clearly a compiler bug, but upgrading the compiler is not an option here. Thus I used unoptimized compilation so far. Now, Cabal 1.2 seems to have optimization set by default. While it was easy to add optimization by using 'GHC-Options' field it is not obvious how to override Cabal defaults. I don't think it was a good idea to switch on optimization silently from one Cabal version to another one. It would have been better to let people configure Cabal globally and manually to use optimization.