Microsoft has been in a kind of "double-bind." It has been trying to use Office integration with Windows Phone and Surface tablets to differentiate those products vs iOS and Android. However they haven't been selling particularly well (save in a few isolated countries). Yet the longer Microsoft held Office back from iOS (and Android) the more it faced the prospect of people getting used to alternative software or (Google) docs in the cloud.
Rumored for a very long time, today Office officially comes to the iPhone in app form (though not the iPad). In order to use the app iPhone owners must be subscribers to Office 365. It also requires iOS 6.1 as well and works on the iPhone 4 and above.
The product appears to require a SkyDrive account in addition but that may be a built-in feature of Office 365. (I'm not a subscriber.)
The new iPhone app allows users to view and edit Word, Excel and PowerPoint documents. However you can only create Word and Excel documents on the app. Users will also be able to edit docs "offline" and they will sync when the connection is resumed (think airplane flight). Microsoft promises that "formatting and content remain intact" on the iPhone and back to the 365 documents in the cloud.
As mentioned, there's no Office for iPad app but that will ultimately come in all probability. For the time being iPad users can access Office 365 through the browser. So effectively Office is available for the iPad.
There are now hundreds of millions of iOS devices in the market globally. This year more tablets are expected to ship than laptops and by 2015 more tablets than PCs in general. In the aggregate there will be more "mobile device" users than PC users in the very near future. Thus Microsoft was all but compelled to bring Office to iOS (Android users can access via the browser).
After Windows, Office is Microsoft's most important and lucrative product -- generating rougly $25 billion in revenue last year. The rise of mobile devices puts enormous pressure on both product lines. However the arrival of Office for iOS means there's less reason to buy a Surface tablet.