Yes, iOS in-app purchases are transferrable across devices that are linked to the same app store account. It is a somewhat strange experience when you do it the first time because you are basically expected to "buy" the item on the additional device and then at the last moment the system determines that you already paid for it and provides it for free -- I had visions of getting a bill for multiple in-app purchases of the same item but in the end it all worked out.
For this reason (and for clean installs) there's an option in the API for "restore purchases". Applications should provide this option in addition to "buy".
Another benefit of it is that it restores all in-app purchases for this app. Very helpful in case the application provides more than one purchase product.
