Google Documents (for instance) makes no promises about keeping your data forever. Instead they act as online areas for the editing of documents. There is no guarantee that your spreadsheet (for example) will still be sitting on Google’s servers in 2074, even if it was still readable then (and I doubt it would be). So although you can use Google Docs to edit your spreadsheet, it is not an option for keeping your spreadsheet when you have finished. You are best advised to export the spreadsheet into another format.

AA: does this mean all the formulas will be lost eh?