I used to think that if I ever got hit by a car while out and about (God forbid, of course), the most useful place to find identifying information would be in my wallet. But that's not really the case any more. As cops are learning these days, the best place in a person's pockets to learn about them is their iPhone. Not only is the phone full of contact information (and indeed, I've included an "ICE" number on every phone I've ever had), but it's also got lots of other information about me, including where I've been lately, what kinds of things I've searched for, and even what apps I've used and how often.
Besides the obvious places (in the apps themselves), there's a lot of information that you as a user don't have access to. iOS apparently caches both screenshots and text used while multitasking and spell checking, so even if you don't actively save information on your iPhone, odds are it's there anyway.
Before you panic and decide to ditch your iPhone for privacy's sake, keep in mind that whoever's trying to get this information needs to have direct access to the iPhone itself. At this point, hackers can't dive into your phone over the air and steal your secrets (most of them, anyway). But this type of "iPhone forensics" is very useful for cops trying to figure out what anonymous victims have been up to lately.