You may have read some communities promoting an Xbox Live app for your iPhone. The app seems really cool because you can easily see your gamerscore, achievement breakdown and see your friends’ cards. The app is also on sale for $1.99 which raised my suspicions.
I would like to use this to warn you to use caution and wait until we get official word from Microsoft about this app. I have made some inquiries on the Xbox MVP boards to get more information. First the app is not sourced from the Xbox Community Developer Program where many trusted community apps are made.
Second, community apps are not supposed to charge for their use. So this app should be free and this person should not be making money off of it.
The largest question is where your login information is going. One of the reviews on the iPhone app page does say that if you have your credit card information on your Xbox 360, then the creator probably has access to that.
Until we get official word from Microsoft about this app, please use precaution and be careful about who you are giving your login information to.