Glad you find it useful! Yes, you can write data to the API via custom views. See Demo 1 at http://code.google.com/p/geo-api/wiki/Demos for an example of writing check-in comments to locations. Full user view API docs here: http://code.google.com/p/geo-api/wiki/ViewUser

Yup, you can write arbitrary data to it in any format you like. See "Writing to a custom-writeable view" at http://code.google.com/p/geo-api/wiki/MethodView.