In 2007, when the Apple redefined the smartphone, Exchange users were able to sync mail, contacts, and calendars with a single Exchange account ... using IMAP, which many enterprises didn't bother to support.
In 2008, Apple held a special event that targeted "iPhone in the Enterprise." During that event, they announced support for Exchange ActiveSync, as well as push email, contacts, and calendars. They also included support for multiple Exchange accounts on a single iOS device.
In 2011, Apple released iOS 5 with a new Reminders feature. Reminders introduced support for Exchange Tasks, allowing enterprise workers to do even more from within the native iOS software suite.
And, in just a few short weeks, Apple will release iOS 7 and add support for Exchange Notes. Read more about iOS 7 and Business features at Apple's website.