Telstra NextG wireless network does not use public IP addresses and consequently the IPSec tunnel cannot be easily setup. You will need to request a static public IP address from Telstra.
Bigpond 3G connections do use public IP address so the IPSec VPN tunnel can be easily setup. However, the 3G connection is allocated an IP number dynamically so you will need to use Dynamic DNS to specify the connection if the 3G connection is the Dial-In connection.
We have tested the VPN connection using the Bigpond BP3 USB modem connected to a 2910 router. We used IPSec with medium (AH) and High (ESP) DES without authentication. The 3G connection was set up as the remote host dialling into the VPN server.