Oh now, this looks like it's going to be a lot of fun! I've been seeing little bits and pieces of yarn bombing around Pittsburgh for a couple years now; an occasional bicycle rack, a tree here or there.
Knit the Bridge is a whole other scale! From their site -
Knit the Bridge is a grassroots, community-led arts project that brings the many diverse communities of Pittsburgh and Southwestern Pennsylvania together to create a large-scale, aesthetically stunning, fiberarts installation on the 7th st./Warhol Bridge in downtown Pittsburgh.Quite ambitious and what a great way to bring the Fiber International to Pittsburgh's streets. Artists are already working on panels in preparation for the installation. The Fiber International exhibit takes place April 19 - August 18, 2013, so I imagine all of the panels and installation will be taking place sometime in late March or early April.
More information about participating, and directions for making panels, can be found on the Knit the Bridge site.