The My Xbox Live app, Microsoft's official mobile link to Xbox Live, has been updated with some nifty new remote control features that allows your iPhone to issue commands to your Xbox 360. It's not a full-featured remote to the point that you can turn your console on, but an included set of nav controls allows you to browse around the dashboard and access Quickplay content.
Connecting the app with your Xbox 360 is as simple as browsing to the "Console Settings" menu and turning on Xbox Companion features, then tapping the associated in-app command. The iPad version was updated as well to now include Retina Display support and improved authentication.
Microsoft also released an Android version of the app, though it doesn't include any of the new features offered in the iOS update; from the look of the product page, it's the original version of the app as it was first released. You can get any of these apps for free from your mobile OS's digital marketplace.
Source: Major Nelson