Office Mobile for iPhone is available for no additional charge for all Office 365 Home Premium and Office 365 ProPlus subscribers. You can sign in with an existing Office 365 account or buy a new Office 365 Home Premium account through the in-app purchase. You can easily navigate between your recent documents stored in either SkyDrive or SkyDrive Pro, open documents from other cloud storage locations, create new documents straight from Office Mobile, and change settings like resetting the Office Mobile app.
Source: Office Blog