They do change, and quite a lot, which means users often have many mobile IP addresses. However, those same IP addresses often show up for many users, so it's hard to know how the counts shake out.
That'll be me...
Daily one hour train commute, and my IP often changes a number of times as I switch from tower to tower, as well as drop out due to no signal in places.
I also switch between networks as my mobile GSM and portable wifi are different providers.
