I tried, but it doesn't make any difference. I can connect to the internet, it doesn't notify me of any problems. But instead of switching to Dual or 3G, it stays in the initial state (GSM) and connects normally to internet. It's as though the auto-switch didn't work. I tried on another N900, it's the same thing. Thanks
gconftool-2 -g /apps/autodisconnect/param_idle_network_mode_type
dbus-send --system --type=method_call --dest=com.nokia.phone.net /com/nokia/phone/net Phone.Net.set_selected_radio_access_technology byte:2