My first thought is that the gateway isn't set, or set right. Being
honest it could be any of a number of things. Can you provide the
output from the following commands and then maybe the problem will be
more obvious to somebody here...
There are other areas (like netfilter) that could also cause your
problem, but this is a good first start at figuring this out.
"You cannot depend on your eyes when your imagination is out of focus."
Bradley W. Olin http://www.yqcomputer.com/