From the Instagram blog:

Today we’re announcing Layout from Instagram, a new app that lets you easily combine
 multiple photos into a single image.

The app I was occasionally using for this purpose was last updated for iOS 6 so it's good timing for me.

It makes sense to not put this inside the existing Instagram app - people are searching for a layout/photo arranging app anyway. Might as try and keep them inside the Instagram ecosystem rather than using a 3rd party app and hope they remember to post back to Instagram.

As with Hyperlapse, Layout by Instagram is available for iOS first in the App Store, apparently available on Android in the coming months.