I've been trying out iwork on the ipad, and tried some exporting via iwork online. It seems to me that it becomes read-only on iwork online, unless I'm missing something. Can iwork online work like dropbox, or is it just to view a document?
The way I used iWork.com with Keynote I was able to share the file from Keynote on the Mac to iWork.com, then open iWork.com on the iPad and send the file to Keynote. It worked going from the iPad to the Mac also. Editing it and re-sharing allowed me to overwrite the online file or save a new version.