It just happened to me that the DNS of my ISP was down. Under a Ubuntu Intrepid Ibex (8.10) in a place where I damn needed the web.
In this case, you just have to ensure that you have a replacement DNS server, for instance, the public 184.108.40.206, which works very well so far. Edit the config file /etc/resolv.conf.auto and add the following lines:
After that, restart the networking service by:
# /etc/init.d/networking restart
and the web works again.