I was having the same problem with a number of sites, including the ones you mentioned and also http://drupal.org .The problem started a few weeks ago, when I changed to a new physical network in a new house.

The problem happens in ubuntu 8.04 with 2.6.24-19-generic Kernel, Ubuntu 7.04, OpenSUSE 11.0 and even on Knoppix (I forgot the version). Basically, I tried all my live cds without success.

At first, I thought it's a network connection problem. But I found it works from Window$ 2003. Then I thought it's Ubuntu problem. I tried OpenSUSE 11.0 to get the same error. Then I thought it's a driver problem, or may be a firewall problem. But couldn't find anything. I tried with wireshark and captured the packets, but nothing came out.

Following your suggestion, I have installed and started firestarter, and the websites are available now.

Thank you so much!!!