There is no way to achieve what you want with the GUI. But with the command line you can select any subset of connection types. The gconv key /system/osso/connectivity/network_type/auto_connect contains the list connection types that N900 connects automatically to. Just remove DUMMY from that list.
Nokia-N900:~# gconftool-2 -g /system/osso/connectivity/network_type/auto_connect [*]
Nokia-N900:~# gconftool -R /system/osso/connectivity/ui/auto_connect /system/osso/connectivity/ui/auto_connect/60_wlan: network_types = [WLAN_INFRA] label = conn_set_fi_conn_set_wlan not_found_banner = conn_ib_net_no_saved_wlan_iaps gettext_catalog = osso-connectivity-ui /system/osso/connectivity/ui/auto_connect/65_gprs: network_types = [GPRS] label = %s /system/osso/connectivity/ui/auto_connect/90_any: network_types =[*] label = conn_set_fi_conn_set_any not_found_banner = conn_ib_net_no_saved_iaps gettext_catalog = osso-connectivity-ui /system/osso/connectivity/ui/auto_connect/01_always_ask: network_types = [] label = conn_set_fi_conn_set_always_ask gettext_catalog = osso-connectivity-ui Nokia-N900:~# gconftool --recursive-unset /system/osso/connectivity/ui/auto_connect/90_any Nokia-N900:~#
Nokia-N900:~# gconftool -R /system/osso/connectivity/ui/auto_connect /system/osso/connectivity/ui/auto_connect/60_wlan: network_types = [WLAN_INFRA] label = conn_set_fi_conn_set_wlan not_found_banner = conn_ib_net_no_saved_wlan_iaps gettext_catalog = osso-connectivity-ui /system/osso/connectivity/ui/auto_connect/65_gprs: network_types = [GPRS] label = %s /system/osso/connectivity/ui/auto_connect/90_any: network_types =[*] label = conn_set_fi_conn_set_any not_found_banner = conn_ib_net_no_saved_iaps gettext_catalog = osso-connectivity-ui /system/osso/connectivity/ui/auto_connect/01_always_ask: network_types = [] label = conn_set_fi_conn_set_always_ask gettext_catalog = osso-connectivity-ui Nokia-N900:~#