An in-app purchase confers the same rights and suchlike as a standard purchase. But it may be confusing to get it included depnding on how you purchased it. For iOS5 the App store has cleverly included informative info such as "install" or "installed" rather than just showing the price of an app. I would assume that apps that use in-app purchases that are fully iOS5 updated would also show such information, but I fear that the purchase mechanism is not necessarily so controlled in terms of what it looks like. Some apps have a "restore purchases" function, others may just list the purchases available but then not charge you when you repeat download an item (on the assumption that is is not one of those "buy 1000 credits" purchases that can be legitimately purchased multiple times) like pre iOS5 app store did.
If you purchase on one device, buy the in-app item, backup to itunes, then use it to sync the app and data (inc. purchases) out to the other devices it should work fine and be less potentially confusing.