As of October 09, 2012, the OnSwipe iPad theme is no longer available on WordPress.com. iPad visitors will be redirected to the 'Desktop' theme of your site. Most of our themes work fine on the iPad, but if you want a theme that works great on the iPad, go to Appearance -> Themes in your blog’s dashboard, and filter for 'Responsive Layout'. Details here:
Responsive Layout themes are those that adapt to different devices like smartphones and tablets. With responsive Layout themes, there’s no need for you to enable an iPad specific version of your site, since it will look good on iPads, tablets, smartphones, and desktop. That's the beauty of responsive themes.