I mean FIFA 2012, really?
I'm bewildered.
That's pretty interesting, so was mine. Also, he played FIFA 2012, got achievements, spent my points, and added friends. The culprits were definitely in Europe. When I got it back I joined a chat room and listened to them.
1) They hacked it IMMEDIATELY after I added $50 worth of points.
2) I DEFINITELY didn't have a predictable password at all.
3) I don't use windows machines, only *NIX.
I had the password shared a few places, so I think one of the forums I visit has been compromised.
Isn't 360 account hacking primarily used to use people's points on content that can be shared with other profiles? Why would he actually use your account actively?

I did contact MS, since it cost me about $50 worth of points. They told me that there would be an interim period of time where I will be able to log into my account and earn achievements, etc. During this time I won't be able to add points to the account or buy content. Once the investigation begins the account will be locked and I won't be able to log.
I'm still in phase 1 and it's been about 3 weeks. Phase 2 is supposed to take 2 weeks once it begins.

Did you contact Microsoft and what has happened so far?
For me it was was a shared password but I wouldn't call it predictable. I actually changed most of my other passwords prior to this to a more random password.
It does look like they changed my Windows Live password, but I've since changed that. Thankfully the password to my windows live account is most definitely not the same as my email password (i'm not that dumb.)
This is what I don't get.
Without my account he can't do shit with the stuff he downloaded. I guess maybe with one of the mod chips out there, but I really don't know.