Hey guys,
Dan’s right, though I suspect you’ll want to use a real domain name. Most systems, when you get them from a hosting provider, are setup with a domain name under the hosts top level domain (and that domain name will work forever, assuming it reverse resolves correctly–all of the hosting providers I’ve been with set this up on the box before they hand it over to you).
But, in cases where you don’t have one, it’s really easy to fix. (And I suppose we can add an additional question to the install.sh script to set it, if it isn’t set, though I wanted it to be non-interactive once the EA period is over, and the initial upgrade warning question goes away.)
Anyway, to fix this problem on your system, run:
hostname ns0.mydomain.com
Where “mydomain.com” is any domain you control. Doesn’t matter which. The ns0 is also arbitrary. It’ll be picked up by several of the services during install, and so should be something you don’t mind showing up in various places. It can be “primary” or “main” or “woohoo” or whatever you like. Or, if you wanted to look like a huge faceless operation, you could use “jr915.mydomain.com”. And folks might think you have a thousand servers and this one is number 915. In short, doesn’t matter what you call it, but by the time all is said and done, you’ll need it to be a “real” resolvable domain. 
Next up, add it to the /etc/hosts file, using your favorite text editor. My entry on virtualmin.com looks like:
70.86.4.238 virtualmin.com virtualmin
Note that all of this stuff can be changed later, and you can use an arbitrary name, as Dan suggested–no harm in that. It’s just that if you don’t want to have to do the work of changing it later, might as well get it right now. The stuff that breaks when you don’t get it right is usually pretty minor, though mail delivery can be a problem, as can the automatic name server setup in Virtualmin. Both are easy to fix, however.
