Well I can't think what you snag might be! maybe because I don't know what you're refering to by "SBC".
I mean, what kind of modem are you using ? The DSL service, is it a "cable" service or what? Does the modem (or maybe a modem/router) connect via ethernet or USB ?
Plus, when you did the install and got the connection, had you configured a fire wall? (because I had lots of probs with that and getting access to the net).
Erm, have you tried opening a terminal/konsole window and trying to ping something? like if you know the IP address of the gateway (the modem or modem/router), have you tried typing
where the x's are the numbers of the IP address for the device? or maybe try
then if it's getting out, you should get replys bounced back (which you can stop with either ctrl Z or ctrl X - I can never remember which