Are you running a web accelerator? These, often times freeware, programs optimize your internet access or blocks pop-ups by means of a HOSTS file? I was experiencing something similar to what you described above, I’d accessed the site many a time in the past, then one day I got an error message. Nothing I did changed it and finally I gave up until one day it happened again to another important site I accessed regularly. Turns out that the IP for the sites I was trying to get to had changed but the program I was using (TweakDUN) was still resolving to the old IP addresses.
The idea behind using the HOSTS file is that if you can resolve IP addresses on your own computer instead of waiting for a DNS server to do it, you can cut the time required to find a Web site. If you have a slow connection or if the servers are very busy, you can indeed shave a few seconds off the connection time of your most used sites. Or on the rare occasion when DNS servers are down, you might even be able to continue to use the Web.