I've been using it for a while. There was a beta version available for free, and I believe most enterprise customers have had access to the final RTM version for months. I know that some MSDN subscriptions include a production license, and I was able to purchase a consumer copy from Microsoft's Home Use program last week, as well.
The RTM update to Office on Windows RT was delivered the week before Windows RT went on sale.
Office 2013 is out. I know because I bought it yesterday.