Microsoft has just released an official XBox Live app for iOS (free, App Store link). The free app is designed for both iPhone and iPad, and allows you to read and send messages, manage your friend list, view your achievements and more.
From MajorNelson:
This free app will allow you to stay connected to the Xbox LIVE community when you are away from your console. Some of the features include:
- Read and send messages to friends
- Manage your friends list, invite new friends
- Read and Edit your full LIVE profile (name, bio, motto)
- Change your avatar features/items with the avatar closet
- View and compare your achievement progress with friends
My XBox LIVE is available immediately as a free download through the App Store (free, App Store link).