Can a site not respond to a ping?
 

I was having a little trouble with internet access yesterday, so the first thing I usually do is ping something like google.com (they seem to be always up). If the response comes back, I at least know that I'm getting to the DNS server and do have some sort of connection.

One site in particular seemed to be having trouble - couldn't connect etc., so when I pinged it, the requests timed out. As expected.

However, today, I was just pinging randomly, and flyertalk.com was not responding. Since it was patently up and on line, I was wondering if it's optional to respond to pings?

Yes you can stop ICMP (Ping protocol). At work on the webservers we don't allow ICMP so you can't ping the site, although the site is live.

Another interesting thing I just noticed:

So, what happened to the "www" in front of the site name?

Some sites block pings, some ISPs block pings as well so you might not be able to ping anything. Blocking ping requests or responses doesn't cause anything else to break but it also doesn't accomplish much either as you can just do a different kind of probe instead of ping to check for connectivity (but it's more expensive for the servers).

Some sites block all ICMP messages which actually causes real problems as they're part of the basic IP protocols. Blocking ICMP errors can cause extremely slow download speeds for users due to breaking the PMTUD protocol.

The canonical name is flyertalk.com. www.flyertalk.com is just an alias for flyertalk.com:
The web server appears to respond to both virtual hosts with the same content.

This is incidentally not a good way to set things up. It means you can end up with two sets of cookies or site preferences or other state depending on which host name you connect to which can be confusing. It's much better to set up a virtual host for one of them that just does a 302 redirect to the preferred name and always use that preferred name.

No, it doesn't. If you go to flyertalk.com it redirects to www.flyertalk.com.

You probably mean a 301, which is exactly what they send when you access flyertalk.com. (302 will work as well, but 301 is better)

In fact, not responding to pings is also the default behavior in Windows since I think Windows Vista (maybe XP).

All of my project's Production servers are set to not respond to pings.