The leaked Windows Phone 8.1 SDK it teasing some tantalizing details about functional improvements to the OS.
With Windows 8 and Windows Phone now sharing a large part of their APIs, the Windows Phone (code-named "Blue") Runtime API Reference appears to suggest Windows Phone 8.1 will support the Windows.Devices. HumanInterfaceDevice protocol, which gives Windows app access to devices that support the Human Interface Device (HID) protocol.
Unfortunately the documentation is still preliminary and subject to change, but I suspect hundreds of Nokia Lumia 1520 owners cant wait to add a keyboard case to their accessories bundle.
Look forwards to seeing more from the leaked SDK over the next few days.