Hi krypto,
Given that the DNS system is designed to do the opposite of what you are trying to do (resolve IP addresses to hostnames) I suspect you are going to have a uphill battle in trying to achieve your goal. Especially if you want your site to be accessible both ways – only via IP and by hostname.
The solution may lie in using .htaccess to direct apache to handle IP requests differently to requests using the domain name, but the actual mod_rerwrite code to do that is beyond me. I suggest you ask your question in a webmasters forum such as stackexchange.com