KT I think you are telling us that you have a static domain. As in your domain is the only domain on that IP address here is an example of what I'm talking about http://www.whatismyip.com/faq/difference-between-static-ip-address-and-dynamic-ip-address.asp if that is what you meant "by some sites (who do things well) 301 on the IP, others don't so I wasn't sure if I was missing something"
I would say the only chance of any site doing well in the search engine world is if it is a static domain obviously gone over that still there are very many security reasons to have a DHCP over static IP if somebody decides to attack your site for whatever reason you are a sitting duck with a static IP at least a DHCP can change it and the way all the search engines are set up now a virtual IP domain name or DHCP is treated just as if it were a static IP to Google there is no difference the DNS domain name service has to take those numbers and turn them into a readable user friendly word we can remember I think you're confused about the difference between an IP and a domain regardless of what you do your site will suffer greatly if you do not do a 301 redirect or some sort of mapping as I suggested and please understand that is a guess I don't know that a robot.txt would be sufficient I would strongly suggest because you're putting so much time and effort into making a website that you want to rank well on the search engine's using a 301 redirect whether or not use static IP or domain on a DHCP here is some more info as well http://en.wikipedia.org/wiki/URL_redirection
I wish you the best with this please do inform me of any way that you have of knowing you will not be victim to the same issues everyone else's when it comes to duplicate the links duplicated content. I do know Google bots don't just stop when they see no follow they go on and they indexed everything then they decide what they wanted for you to rank well. I think it would be more than confusing when you type site:example.com and site: www.example.com into Google only to find you spent time building links for one of your two websites.
Regardless I wish you the best on your endeavor,
Thomas