Yeah sorry I'm not sold on it being an 11 year old or that someone else could have easily done something much worse.
I'm hard pressed to believe this was an 11 year old for 2 reasons:
1) iPhones are damn expensive, what is an 11 year old doing with an iPhone?
2) I work with xml every day in a professional environment and can't see how I'd be able to trash an iPhone using xml.
3) xml is a type of database, not a programming language. I'd be impressed if an 11 year old could get an rss feed going using an xml database let alone hack an iPhone using it (apparently).