I was confused by this for a while, until someone clarified that the analogy isn't that great. I prefer to think of 'ser' as a characteristic, and 'estar' as a state of being. One's newness is a function of their character, an attribute used to describe them that won't really change day to day, so 'ser' feels more appropriate.
This isn't a perfect analogy either, mind; you'll ultimately just need to practice with the language more and get a natural intuition for when to use which form, but I find that this analogy holds up in more situations, where the "permanent / temporary" analogy tends to break down more readily.
Actually, it should be "Thou art new in the neighborhood." Unfortunately, English speakers destroyed their intimate form rather than extend tolerance to those pesky Quakers who were going around calling everybody "thou" and "thee" in a vain effort to get everybody to be kinder and friendlier to their fellow human beings.