I think you might be jumping to conclusions about what Microsoft is going to allow. The Xbox environment isn't exactly known for being open. My guess is that mods are going to have to submitted and approved from the developer's side, after they've made sure they work. Look at the certification process they have to go through for patches, I can't imagine they'll let through hundreds of poorly written mods, some of which will almost certainly be malicious in nature.
This isn't going to be like the PC, I'm almost positive.








