It looks like Google Analytics isn’t tracking Bing as a Search Engine yet, it’s currently being tracked as a referring site. So if you’re interested to see how you’re doing for certain terms in Bing, then you’ll have to wait until Google catches up.
So if you’ve seen a spike in referral traffic since the 1st of June, then the reason this is probably the reason.
The same type of thing happened after the launch of Google’s own browser Crome. It wasn’t tracking people’s use of Crome at the start… which was a bit of an oversight as it’s their own browser.