[contingencies]

Logging on to GSM your phone uploads its IMEI ('phone serial number') with the SIM card identifier or IMSI ('sim ID'). So if you switch, you are still tracked. Budding spies will learn their tech before putting their ass on the line.

[sporkologist]

As mentioned further up these comments, IMEI's can be forged, and phones produced "off-hours" in the factories often have IMEI's consisting of all 0's or all 1's. I gleaned this much just from reading this thread.

[contingencies]

Sure. Also, according to recent quotes from Snowden, this is the primary mobile communciations key the NSA maintain records and search by... not phone number.