Hi Axel,
Take a look into Indirect Configuration (search on the MSDN website).
It requires using an Environment Variable, but this is generally
accepted as the best way of managing configurations. I would also
recommend storing your configurations in a table rather than in
files. It's generally just easier to manage, and deployment can get
messy if you try to deploy multiple packages that use the same config
file using the deployment utility (well .. the deployment utility is
just generally flaky in my experience. I last used it on the CTP
though, so maybe it's improved since then

)
Good luck!
J