Brandon Sanderson recently revealed (via Facebook) that he's been working on a Mistborn short story or novella during his break before beginning work on the final Wheel of Time novel in January. However, the book has gotten a bit bigger than originally planned and is now a short novel of approximately 60,000 words (maybe longer when finished). Tor have bought the book and will be publishing it in 2011, meaning that 2011 will not be Sanderson-less as previously thought.
Interesting news. The new book will be set roughly halfway between the Mistborn trilogy and its planned sequel series, which will be set several centuries later when the people of Scadrial have achieved a level of technology similar to that of contemporary Earth, but with allomancy still in use. From the sound of it, the short story will be set at a time when firearms are in use, possibly a Wild West level of technology (apparently it features the 'best allomancy gunfight ever'). A release schedule or title for the book has not been announced yet.