You have no idea how long I've been waiting for this feature. Currently Instagram appears to be testing it with select Android users, but I am unnaturally excited for the feature to arrive on iOS. I would happily sign-up for an opt-in beta if there was one. It would come in handy for switching between my personal account, my work account, and my BostonPolaroid account.
Instagram Starts Rolling Out Multiple Account Support On Android | Android Police