As noted: no IP is set in stone. Even one of the root nameservers is renumbering in the next few weeks. That said, the nameserver IPs haven't changed in some time. Odds of them changing any time soon, pretty low.
That's what I was looking for, thanks Kcochran.
No plans to change them anytime soon. As I mentioned, I know unforeseen things can come up later down the road obviously.

I'm betting if I setup say ns1 through ns3's IP's to ns1.mydomain.ext and ns4 & ns5 IP's as my ns2.mydomain.ext that even if something changes one or two of them down the road I'll have time to update them before I'm without any DNS.
I'd likely use at least 2 of my servers in the mix as well. I just hate to solely rely on VM's to handle DNS, if the ESXi servers crash or lag that could mean total DNS failure.
I've got several people on Google Apps for email for example, so I have outside services tied to my DNS. Not in an environment where I can spread out to more of a cloud based setup for redundancy right now. So it's just a small hand full of stand alone ESXi servers with local storage. Using the HE DNS for backup would be a nice bonus, was looking in to signing up for either Cloudns or Dnsmadeeasy or something until I found you guys had a setup running.
I'm assuming it will go premium at some point, but at least I know I can trust HE to keep services running.