I recently submitted my iPhone app for review. I also approved the in-app-purchase for my app (and provided the in-app-purchase screenshot). It's a few days later and the app status has been updated from "waiting for review" to "in review"... while the in-app-purchase status is still "waiting for review". Could the app go live in the store before the in-app-purchase? If so what happens when the user tries to make the in-app-purchase? Anything the developer can do to remedy this?
Set the app availability date to a time in the future, then when the in-app purchase is accepted, set both dates to the present.
You should also uncheck the "Cleared for Sale" checkbox in the In-App Purchase page in iTunes Connect. This means the IAP won't go on sale (even after being approved) until you decide it should.
You should have a check in your app to see if an IAP is actually available, and only display it if it was found in the App Store.
The same thing happened to us on one of our submissions. The Apple reviewers wanted to see our in-app-purchase experience before they approved the app. So in the course of the review they made sure the in-app-purchase was up to par, and approved them both at the same time.
I'm not sure if this is the standard among app reviews, so I'd play it safe and go with @nevan's suggestion as well, because if the app goes through without the in-app purchase items enabled, the purchases will fail.