many of the large mobile (internet) providers use huge NAT to manage internet access for their customers. I recently was doing a forensic job and was surprised to learn that some don't keep any track of their IP assignments to "handsets" against time/source port.
So if a web connected resource is trying to establish who was using an IP address associated with a 3G connected device (direct; no VPN etc) then some telecoms provider will not be able to satisfy their request simply because they don't store the information - at all.
This is interesting information for those that wish to have privacy online but what is not clear is exactly which mobile providers do this. Also, with the 3G provider that I dealt with (through a legal team) some of their devices/customers are monitored for IP address assignment and others aren't - e.g. the prepay USB 3G dongles are monitored, but prepay handsets aren't.
good for privacy - but a bit frustrating when you're an investigator!