Since the launch of Bing, Microsoft's share of the search marketplace has more than doubled. Add to that the deal between Microsoft and Yahoo for Bing to power Yahoo's organic results and Bing powers over 25 percent of search.
With the Microsoft/Yahoo alliance also came the affect that Yahoo's paid search platform would be used to power both Yahoo and Bing's paid results. While this may not seem like a big deal on the surface, it is actually huge.
Where once business owners and marketers had to consider whether it was worth the hassle of managing both a Bing paid campaign (for the significantly lower traffic they yield over Google) and also make the same call on managing a Yahoo paid campaign – the two now are manageable in one convenient location, significantly reducing the time it takes to setup and manage.
This of course makes the cost for these campaigns less expensive and when you combine that with Bing's increased market-share then they're in a position to take some of the ad dollars from Google (or at least, gain some for themselves).