The latest version of the iPhone and iPod Touch operating system is expected to arrive today (June 21). This video shows someone demonstrating iOS 4 on their iPhone.

It’s been over a year since iPhone OS 3.0 was released, so iOS (its new name) is overdue. iOS 4 is a free upgrade.

iOS 4 may be free, but it does have certain requirements attached to it. Most importantly, it’s only available to owners of the iPhone 3G and 3GS, and/or second-generation and third-generation iPod Touches.

If your phone or iPod is older than that then you’re not invited to the party. You must also have iTunes 9.2 installed.

Later this week, the iPhone 4, with iOS 4 pre-installed is set to go on sale. And I’m sure many of you Apple fanboys will be upgrading. But for those who aren’t, iOS 4 at least means a partial upgrade to your hardware.