In the 3GPP TS 23.060 specification, we can analyze all the package domain states of the GMM Layer (GPRS Mobility Management).
The package domain states are simpler than radio ones (only 3 states):
The handset has a PDP context established and is able to send and receive data.
The handset isn't transmitting anything but the PDP context is not closed, so it maintains a valid IP address.
In this state the handset don't consume any resource but the network is maintaining his IP address as a valid one, so it's very important to try to maintain the handset in this state in order to be able to Wake Up it and change to a PMM_CONNECTED state in order to transmit/receive information.
In this state, the handset hasn't a PDP context established so it hasn't a valid IP address.