Given the smaller size, would swim around quite a bit in the sleeve I've been using for my rMBP. Haven't seen any case/sleeve announcements. Anyone else? (Specifically looking for 15")
One thing I noticed with a 13" in a 15" sleeve that i was surprised it wasn't actually flopping around so much, just beacuse they have some weight to them. That said, it was a proper padded pouch, and that helped.
I think if you have a 13" sleeve and get a 13" new macbook pro that it's not really an issue.I wouldn't buy a new one until you had chance to test the new mb