Luckily the app will still work without the Pipboy...though that does kinda diminish it's charm.
I'm holding out hope that maybe after the game is released, they'll put them back on the market. Right now, it's a cash in for the special edition, so rarity is important from a marketing standpoint.
On the up and up, there are many tutorials on how to make a Pip Boy IPhone case, if you're feeling crafty.