We've recently started using package configurations in our Development/Test/Production environment since developers cannot access test and production.

We used the process outlined by Ray Barley http://www.mssqltips.com/tip.asp?tip=1405 that uses an Environment variable to point to an SSIS Configuration database.

I'm sure there is a benefit to using XML configuration files, but at this time we chose the database method instead for storing our configurations.