trouble is these youngsters have disposable time on their hands to fritter hours away on xbox live, often in their bedrooms away from the watchful eyes of parents. We tend not to view xbox live as dangerous as the internet and in some respects it isnt; but there still is the propensity for naive kids to give away personal information.
I tend to only play online with friends from work etc. due to the reasons you mention. I dont own MW2 but I imagine it to be a nightmare as its the new big thing and everyones on there. I own COD4 and thats bad enough!