For Gs implementation , i have heard that some phones are not able tu support it, here more explanation: when MS is GPRS attached and SGSN perform detach (after detach timer expiry), when mobile goes from one lac/rac and another he still make it on GPRS channel but no connection so he will move to another location and the network still have the old location. Anyone has more details about this phones (not 3gpp standardized) and procedure.