Unlike other members of the Google Docs family, Google Presentations doesn't have an export function so you cannot download your edited PowerPoint slideshows out of Google Presentations in PPT, PDF or any other popular formats.
Update: This is no longer required, you can now easily export PPT files from Google Docs.
Google Docs does allow you to download the presentations as a ZIP, which is a mix of HTML, CSS and JPGs, but again that's not a very useful or convenient format.
Considering that PPT is such a universally accepted presentation format, it is quite safe to assume that PowerPoint export feature will meet Google Presentations very soon.
But if you are getting desperate to get those PowerPoint slides out of Google Docs now itself because the boss wants them, the good old Print Screen key can be your friend.
Open the Google Presentation file in the browser that you want to import back into PowerPoint - then hit the "Start Presentation" link on the top left (or hit Ctrl+F5) to play the slideshow in a new window. As the slides progress in the browser, hit the Print Screen key each time, add a new slide in PowerPoint and insert the clipboard contents by pressing Ctrl+V.
It's not the most practical solution because the slides are now still images and therefore non-editable - but then something is better than nothing.
The print screen technique can also be used for sharing presentations created in Google Docs on any of the PowerPoint hosting services like SlideShare, Scribd and more.