With "In App Purchase" said to be being "rapidly adopted" by iPhone developers, Apple has announced that such functionality will now be available to be built into free apps, rather than just paid-for.

This will mean that free iPhone and iPod touch applications will soon see content, subscriptions and digital services offered on paid-for basis within the initial free app.

As well as addressing potential piracy issues with verification, Apple says, in a recent email sent out to registered iPhone developers:

"You can also simplify your development by creating a single version of your app that uses In App Purchase to unlock additional functionality, eliminating the need to create Lite versions of your app".

Commentators suggest this development might mean an explosion of try-before-you-buy action on the App Store, with apps free to download, but requiring payment after a set time, or in order to access full content.

There's more info for devs over at the App Store Resource Center.