hey Nat,
Welcome to the joys of SSIS...
I had the same error thrown in my agent yesterday, and I was adament that there was something wrong with the way in which I implemented a custom component, but it turned out that I saved the package as encryptsensitivewithuserkey (once more, thank you Michael, you certainly saved my bacon with your response).
I would suggest that you use SSIS configurations, which means that you dont need to save sensitive data in the package.
Good luck
~PD