Here's one of their promo videos: http://www.youtube.com/watch?v=7xX-B2wG1e4&feature=playe...
More seriously, the pen is pretty fantastic for things like reviewing papers. Having used tablet PCs for years and now having used an iPad for a while, I can say it takes significantly longer to annotate PDFs via iAnnotate or goodreader and either my finger or the iPad keyboard than it did with Acrobat and the pen+tablet pc input panel.
Not that this matters a whole lot - just adding my own data points to the discussion. :)
Contrast the Slate's ad to the overly-aggressive critique of Nokia's advertising for the N97 at http://www.youtube.com/watch?v=vJpEuMidcSU