Comment 1 for bug 278485

I can confirm this behavior on two Intrepid RC machines.

I guess the reason is that the 3g connection doesn't replace the default route upon connection. Adding an option like "Replace default route upon connection" (or preferably "Use this connection to access Internet through while connected") to the network connection options and the configuration guide would solve this issue.

Currently, the only way to gain internet access through a gsm connection in nm is to disconnect from wired and wireless networks and then connect...
(There may be some way to accomplish this by modifying the ipv4 routes settings for the connection, but I wouldnt even dare guess how.)