When you open a data connection after the idle time has expired. Thats to say.. if you open a data connection.. you get an IP... and that IP will be assigned until the session expires due to inactivity. I cannot tell how long that timeout is. You will also notice often you get the same IP over and over in the same area. i.e. Semi-Static. : )
Can always use http://www.ipchicken.com to check what your phones ip is. There might be other sites one can use, but the above works for me. Also remember for doing "other stuff" you normally need an unrestricted APN.