"Jason Dagit"
Ubuntu seems to be a bit behind then. The current official release of the 6.4 branch is at 6.4.2. Debian seems to provide this version, maybe you can use the debian package? But, if I were you I wouldn't worry so much about upgrading ghc but instead upgrading Cabal which is much simpler and takes just a second.
I've asked them to backport 6.4.2 for dapper, but they are reluctant as it would require backporting all third party software (or at least libraries) as well. I filed a request to backport it, but for some reason, I am unable to find it again. If you can make sense of the hopeless launchpad user interface, feel free to add you voice to the choir.
You're cabal version is too old then. Try updating either Cabal or GHC.
It's the latest version (6.4.1) packaged for Ubuntu. I'll have to download and install a newer version manually. Unfortunately, the download site seems to be down again.... :(
The currently easiest way out is to install a binary snapshot. It would sure be nice to have a working ghc as part of the base distribution, with automatically upgrades etc. Let's hope they put a stable 6.6 into Edgy. -k -- If I haven't seen further, it is by standing in the footprints of giants