There are many great reasons why you should move to VSTO 2005 for your Office 2003 automation: host controls, excellent databinding capabilities, flexible and easy access to the document actions pane, and many more. The ability to cache your data as a data island in the document and make it available at runtime is another great new feature.
Beardie has a small sample of how to store a DataTable in the cache and access it at runtime. Nice to start with.