The IP address is valid. I have used it to send emails to my gmail account. I am using arduino UNO and the shield is connected through RJ45 cable.
Inclusion of '&Serial' as below doesnt affect the output anyway.
int status = twitter.wait(&Serial);
The statement ' if (twitter.post(msg))' returns true which means it is able to establish connection to twitter.
The problem lies in the posting process. Should return HTTP status code in response from Twitter, e.g. 200 - OK. Only available after posting the message is done. But in my case the status is 0 rather than 200. :~