Go to options, type mobile network, select the only option available, change "Network Technology" to GSM/UMTS. A mew option will appear for Network Mode, where you can select 2g, 3g, or both. As far as I know, that's the only way to do it.
Of course, then us folks on CDMA get an SOS message on the signal
You can also access mobile options if you click on the dropdown thing for signal, wifi, and all that other stuff.