I'm making a prediction here. It's not based on hard data, although the data is there showing an upward trend. Rather this is going to happen largely as a result of dissatisfaction with Google. The Google brand is not gaining shine, but rather losing it. When you see a trickle to Bing, you're going to see that trickle lead into something much larger.
I also see the search engine integration in mobile devices as another reason. Microsoft Bing will be in tablets and smartphones and the Microsoft platform will be condusive to Bing usage. It will certainly have more exposure.
Whatever the case, all Bing really needs to do is essentially copy the Google skin and that could be one of the few things standing in the way. I think there is growing issues with Google right now and it's getting worse before that gets better. It spells good things for Bing.
I bring this up because for the first time almost ever, I've had to start using Bing as a comparison for search results. I've never had that need in the past.