iOS 4.1 is live, get to iTunes and hit the update button. If you're already on the GM, you'll likely need to hit restore instead of update. If you're jailbroken, you might want to stay clear or you'll lose your Jailbreak.
It's reading as 589.9MB for iPhone 4.
For more, check out our complete iOS 4.1 walkthrough, and iOS 4.1 in 10 minutes quick start video (below).
Let us know if you've got it and how it's working for you on your iPhone or iPod touch!