Several weeks ago I wrote an article about SSIS configurations that got published in MSDN: Understanding Integration Services Package Configurations. The article covers all available types of package configurations, its differences and some other facts that would help you on making the best choice when adding configurations to you packages. Mine is just one in a set of articles that a group of SQL Server MVPs put together to concur with the launch of SQL Server 2008.
You can find all articles here: http://msdn.microsoft.com/en-us/library/cc872864.aspx
I hope you find them helpful!