While the collaboration functionality of Box.net appears good, and it would be very useful when doing group editing, I think I would still use Slideshare when saving and presenting the final version of a Powerpoint presentation. See my blog posting at http://bibliothekia.blogspot.com/2008/04/slideshare.html for details.
While looking at Box.net it was also interesting to see the following information on their About Us page.
"Box.net is the first web-native system for access and collaboration which allows a broad array of functionality. With nearly 2 million users, over 1 million files served every day, and more than 1000 developers in the Box Enabled Network, the service is used by individuals, small businesses, and Fortune 1000 companies."
Learning 2.1: Online File Storage with Omnidrive