The API documentation states
Use DocumentApp.getActiveDocument() to access the document that is currently opened by the user, if there is one (null is returned otherwise). An active document will be available if (and only if) the current script is installed (or otherwise saved) in the scope of an existing document.
So far I am able to install a script in a Spreadsheet and manipulate any Document with it but I wonder if it is possible to install the script directly in the Document. This would have the advantage of having the script menu right where it is needed. The spreadsheet solution seems like a strange workaround to me.
Add issues/feature requests here:
http://code.google.com/p/google-apps-script-issues/issues/list
DocumentApp.getActiveDocument() looks like the suggestbox issue, i.e. the docs exist but the feature is not usable. Is a good way to check if demand exists for a feature.