Thanks for that Christian. What you've said makes sense. I'll have to think about which one I'll use.
If I may just check my understanding, if I want to be able to add subsequent zones through the ISPConfig control panel after I've created my main my main one, then Ishould create the zone mydomain.com on Rackspace, using their nameservers, then create ns records, ns1.mydomain.com and ns2.mydomain.com (ns2 being on a different server). I can then create my new domains myotherdomain.com using ISPConfig and get it to use ns1.mydomain.com and ns2.mydomain.com as it's nameservers?
Thanks again for your reply.