One feature of Apple's new Messages beta that we missed on first inspection is its support of drag-and-drop file transfers. The new app allows you to add files to iMessages, which can be sent to an iOS device or other instance of Messages. MP3 and video files can be played, text and PDF documents can be read or opened in other apps by tapping the Action button; everything works exactly as you'd expect it to.
Transfers are a little buggy, with text files sometimes freezing up, and we hit a file size limit with a 220MB MP4 video. Incompatible file types are represented with a question mark icon on iOS devices, and can't be acted on. Also, transfers don't work the other way around, so there's not yet an easy way to send a PDF from an iOS device within iMessage, or via iMessage from within iBooks, for instance. Perhaps we'll see an update to the service when the Golden Master of iOS 5.1 arrives in the next couple of weeks.