Forget the hack, Google Docs now lets you export presentations as PPT files in addition to supporting the PDF format. You can also view your Google Presentations offline in the web browser using the Google Gears extension.
To save a Google Presentation in PPT format, go to the File menu and click "Save as PPT". The export feature has been simultaneously enabled both in standard Google Docs and the Google Apps version. From Google Blog and What’s new.
Related: Where to Upload PowerPoint Files?, PowerPoint Tips