Yes, you are right, but using "ghc-pkg list", as suggested by Christian Maeder, I've found out that I've to remove also the ~/.GHC folder.
Deletion of this folder solve my problem.
 
Thanks.
 
Luca.
 
> From: ehamberg@gmail.com
> Date: Wed, 20 Apr 2011 09:51:02 +0200
> Subject: Re: [Haskell-beginners] (no subject)
> To: luca_ciciriello@hotmail.com
> CC: beginners@haskell.org
>
> On 20 April 2011 09:35, Luca Ciciriello <luca_ciciriello@hotmail.com> wrote:
> > Now I've installed GHC again, but when I try to install the package hsgsom
> > cabal tells to me that the package is already installed and I have to use
> > the --reinstall flag. So, where the information of the installed packages
> > are stored on my system?
>
> I'm not familiar with Mac OS X, but at least on other Unix systems cabal
> installs (local) packages into ~/.cabal – i.e. the “.cabal” directory in
> your home directory. Removing this would mean “starting with a clean
> sheet”.
>
> --
> Erlend Hamberg
> ehamberg@gmail.com