You'll need a free OnLive account to sign on to the service, and you'll need to buy LA Noire on the service if you want to play the whole thing (though just like all of the other games on OnLive, there is a free trial available to play with as well, no installation required).
And for now, you'll still need an Android tablet. OnLive has said it's working on an iOS version, but even though the company has released an iOS app for its Windows PC cloud service, the game client is "still in approval process with Apple," the company told Joystiq. It's been in that situation ever since it was announced, which suggests perhaps Apple isn't so happy with the app, though OnLive couldn't confirm anything along those lines. At any rate, if you have the app on Android and want to try touching LA Noire a little more directly, give it a shot.