I read in easyDNS that, in order to set up reverse DNS, "The IP block must be delegated to our nameservers by your ISP." Is that always the case? What if you decide to host your name servers? Will adjusting the PTR records suffice? Some discussions on the web lead me to think that it isn't.
After these boolean questions (in particular if the answers are negative), please feel free to elaborate the DNS 101 concepts behind them. In my no-background mind, it looks like you are "given" that IP address (say, by your VPS hosting company), and that you should be able to "tell the world" that, having reached that IP, it corresponds to domain example.com.