I think we didn't include this feature deliberately for some reason. Isaac can you remember?
It was possibly because I objected to it; although now I can't remember exactly what it was I diskliked so much about it. Possibly just the fact that I think this is the job of the system's package manager, not Cabal. But the same argument applies to 'setup install' of course.
So to be clear, I don't object to uninstall any more, if someone wants to go ahead and implement it that's fine by me.
Great! Having the system's package manager take care of packaging has not work well for the Ruby on Rails community (with Ruby Gems). Lots of Gems are either missing or outdated since the system's package maintainers can't keep up with the explosion of Gems. I often find myself in need of the latest version of a Cabal package (right now I need the latest HTTP) only to discover that it won't be available until the next release of my distribution (in this case Ubuntu Edgy).