When you order your reseller hosting, if they offer it, you should receive 2 IP address's. Please Note: Not All Domain Registrars Offer Domain nameserver registration.
If yours does, Login to your Domain Control Panel, and go to the area where you Register Nameservers. You will be asked to enter the name of the Nameservers, which if your setting this up for hosting or something like that, should be NS01.yourdomain.com and NS02.yourdomain.com. And then enter the 2 IP's into the area asked to.

I hope this helps everybody.
Mark Eastwood.

Missing a few points. Like forgetting the obvious in that checking if the two IP's are on the same netblock in the same datacenter which is a case in worst practice for DNS hosting and spells no planning or foresight.

Or remembering that the PTR record if the hosting company has done their job will never return a record that the server is yours...

But those are minor points :-)

-Roger