Microsoft has released My Xbox LIVE for the iPhone, iPad, and iPod touch.
Take your Xbox LIVE experience wherever you go with the My Xbox LIVE app. Track and compare your achievements, connect with your Xbox LIVE friends, and change up your 3D Avatar. Review all your recent great games you and your friends love to play and compare achievements with them. Jump into our games hub to learn about the latest LIVE games and apps. Access Xbox Spotlight feeds, get breaking news from Xbox LIVE, game tips and tricks, gamer spotlight and much more.
You can download My Xbox LIVE from the App Store for free.
Sounds like you're jailbroken (and unlocked). You have to be jailbroken and on a mac for this to work. I'm not sure how you'd do it on a PC
I've uploaded the app to Mediafire. Download http://www.mediafire.com/?jvn2auhaarxzsi1 and try this:
1) Change the extension from .ipa to .zip
2) Unarchive the zip
3) Open the folder "Payload" (or "My Xbox Live 1.0/Payload")
4) Right-click "MyXboxLIVE" and click "Show Package Contents"
5) Open Info.plist with any text editor or Xcode
6) Find the part that says "MinimumOSVersion4.3" and change it to "4.2.1"
7) Go back a couple folders and select all of the three files "iTunesArtwork", "iTunesMetadata.plist", and "Payload"
8) Right-click one and click "Compress" or "Compress 3 items"
9) Rename Archive.zip to "My Xbox Live.ipa"
10) Then either install the .ipa with iTunes, or put it on your phone into the folder "/var/mobile/Documents/Installous/Downloads" via SSH, Fugu, or the like, and install it with Installous
11) Pray
Hope this helps! It's a bit convoluted, I know. I'd do it myself if I had a computer with me.