I don't suggest running them both on the same machine; even if you bind the slave to a separate IP and have it setup correctly, you know you have this single point of failure.
I've used ZoneEdit, and GraniteCanyon in the past. ZoneEdit's interface is a bit hokey, but their service works. If you want to pay someone too much money to host a zone, there's always UltraDNS.
Edit: Ray, hit me up when you have a second? I need to do some stuff to StrongBox, and I'm wondering if you'll support it, if I do.