As I previously mentioned, SQL 2008 R2 added new Bulk Insert functionality to the ADO.NET Destination. This changed the component version, which means that if you edit/save packages that have ADO.NET Destination components in R2, the package will give you a loading error when you use it on a 2008 (R1/RTM) machine.
SQL MVP John Welch has a very good write up of the issue, and provides steps (as well as an XSLT script) for “downgrading” your package to the 2008 format.
Note, this problem only happens with packages that use the ADO.NET Destination. All other packages can be moved between R2 and 2008 without any problems.
Also note that even without modification, the package will load in the 2008 designer (with errors). Replacing the old destination with a new one (and re-connecting the metadata) will resolve the issue.