The build 20 feature set here.
Lots of good stuff but in particular these two:
iPhone support DPS now supports the iPhone 3GS, 4, and 4S (the iPhone 3G is not supported). When you use the Viewer Builder, you can create a custom viewer that works for the iPhone, the iPad, or both. When you create an app for both the iPad and iPhone, your customers are entitled to purchased folios on any supported iOS device. The iPhone viewer includes a new settings page, enhanced browse mode, and welcome screen that publishers can control. Social sharing, bookmarks, and auto archiving are not yet supported for the iPhone. At this time, you cannot create a single-issue viewer app for the iPhone. See Creating content for the iPhone.
Web viewer for social sharing If you enable social sharing in your app, representations of folio articles are uploaded to a web server for viewing in desktop browsers. Use the Protected setting in the Folio Producer Editor to determine which articles are available for desktop viewing. Use the Account Administration tool to limit the number of viewable unprotected articles and to set up a paywall. See Using social media.