Symphony.

I'm organizing the different interchangeable parts of my projects into site applications (e.g. weblog, newsletter, image gallery, etc.) , and each of these uses its own standard set of Sections and Datasources. I'm trying to come up with a convenient and organized way to define the data model for each of these apps. Currently, I'll just be using it for reference as I manually re-create the Sections & DSs in each site, but I could see it being used programmatically in the future (to allow for installing the app into the site automatically). I'm thinking of a XML schema or something similar.

Has the Symphony team (or anyone else for that matter) already done anything like this? I'd like to avoid reinventing the wheel, and also keep my stuff as close to any potential future core compatibility as possible.

I don't think there's anything like this. We haven't done one ourselves. We've thought about this briefly in the past but we don't have any plans in the near future to develop this though. I don't think this would be a difficult extension to create. It'll be a matter of parsing the source and mirroring the SQL of Symphony's section and fields.