Future: Bidding on TV ad spots?

It works on the Internet, why not Print, TV & Billboards? Rather than having set rates, why not have advertisers bid for a spot. A Google billboard could be available to the highest bidder, and if it was digital, ads could change instantly. At MediaPost they are asking:

Imagine for a moment that Google cracks the code and extends its yield-managed auction not only to broker print, but also radio, television, out-of-home, and new media formats like podcasting and handheld devices.