I've been wondering why Apple hasn't implemented pull-to-refresh in any of its apps. It is a fundamentally better user experience for refreshing time-based list items. As it turns out, patents may be the issue. In a patent titled “User Interface Mechanics,” owned by Twitter and listing Loren Brichter, the developer of Tweetie (which Twitter purchased), among its inventors, the abstract describes pull-to-refresh behavior:

Input associated with a scroll command may be received. Then, based on the scroll command, a scrollable refresh trigger may be displayed. Subsequently, the scrollable list of content items may be refreshed in response to determining, based on the scroll command, that the scrollable refresh trigger has been activated. In at least one instance, it may be determined that the scrollable refresh trigger has been activated in response to determining that the scroll command was completed while the scrollable refresh trigger was fully displayed.