This is being released as an add-on to the existing SDK, so developers can make sure their apps will be up to date with the latest and greatest.
In addition to the host of features which we told you about earlier which will now be available to developers, Honeycomb 3.1 will include the following new and updated user features:
Numerous enhancements to the UI to improve efficiency and make the interface more intuitive. Improvements to launcher animations, ease of visibility and improved audible feedback further add to ease of use.
Connectivity for USB Accessories
Support is now added for many more USB devices, including, keyboards, mice, and digital cameras. Virtually any USB peripherals can be supported, by applications built on top of the OS' native USB support.
Expanded Recent Apps List
The recent apps list now retains many more items, and includes "touchable" thumbnails.
Resizable Home Screen Widgets
This sounds like a feature which users of alternative launcher apps are already familiar with, now native in Android 3.1.
Support for external keyboards and pointing devices
Including scroll wheel and trackball devices.
Support for joysticks and gamepads
Yes, this means your PlayStation and XBox Controllers.
Robust Wi-Fi Networking
Speed and performance improved, especially for streaming media
Updated Set of Standard Apps
Browser - UI improvements, added support for standards like CSS 3D, animations, CSS Fixed Positioning, and HTML5 video playback. Pages can be saved for offline viewing.
Gallery - Now supports Picture Transfer Protocol (PTP), for one-touch import of pictures from external digital cameras.
Calendar - Larger grids, better readability and ease of use.
Contacts - Now allows full text search of all fields.
Email - improved compatibility for HTML email, IMAP improvements, enhanced Home Screen widget.
Includes improvements to HTTP proxy and Wi-Fi access.
Let us know if you're developing apps for the new 3.1 platform, and what your experiences are.
Source: Android Developers Blog