Check Iphone Ip Address

The wireless carrier probably keeps track of that information. And they're not likely to give up that information without a court order. I would think that it's an arduous task to track down a specific connection, both from a technical and legal standpoint.
One of my apps is able to show me the iPhone's current IP addresses, both for WiFi and Cellular. On the AT&T cellular network, the IP address is always a 10.x.x.x address. Meaning, it's in private (not internet-routable) address space, and AT&T is using Network Address Translation (NAT) to provide internet access to my device. This doesn't surprise me. With IPv4 address space getting tight, a huge number of iPhones on the network, and the vast majority of apps not needing to listen for incoming data, there's no real need to assign a public IP address. There are available protocols like UPnP for the rare times when an app needs to receive data from an unsolicited source. I don't recall if the iPhone SDK supports UPnP.
This is all speculation, but I imagine that the carriers keep track of which private IP address is assigned to a subscriber and at what times. The carrier's NAT service would also need to keep track of which private address connects to which public address and port. Correlating those together, an individual connection could be tracked to an individual subscriber. That's a lot of data to retain, so it's probably not kept around for very long. Again, this whole paragraph is speculation. Whether or not they do any of this is another matter. I'm just pointing out that it's definitely possible.