The most common cause of this problem is a miss-configured local DNS server. Here's our current blurb about this problem:
A common miss-configuration of software that causes these packets to be sent to localhost.com is the configuration of your DNS server. Try running ping, traceroute, and nslookup for the unqualified name localhost on the machine in question. None of these should go to localhost.com or to any machine outside your local network. If you see these packets escaping, you will probably need to add localhost to your domain, and may have to configure any machines that think they are XXX.com to look at your local DNS server first, instead of going to the com DNS server.
To solve the first problem, place an A record in your DNS tables for a localhost in your local domain, with the address 127.0.0.1.
For the second problem, you may have to add a line to resolv.conf saying "domain XXX.com".
Another way of accidently getting to the localhost.com web page has been pointed out to me by Tom Rathborne:
Here's another reason that people can end up at www.localhost.com: Netscape Communicator 4. When you enter a single-word hostname like 'localhost', it (sometimes) pre-emptively turns it into 'www.localhost.com'. Everything else on my machine (and any nameserver I ask) agrees that 'localhost' is 127.0.0.1... but Netscape doesn't bother asking, diddles the URL, and sends me to your page instead.
A number of people have pointed out an additional source of trouble. Many linux distributions start a web server on boot these days, and if that web server fails to start for some reason, trying to connect to 'localhost' may end up at localhost.com. The same problem can be try for a Microsoft operating system as well. If you expect to hit your private web server, but instead hit localhost.com, make sure your private web server is running.
Brought to my attention by the nice Juan Cabello (email not given since I didn't ask permission). If you force all your web broser traffic through a proxy and don't exclude 'localhost' from the proxies, you might not get back what you expect when you try to look at your local web server. :)