Registrars have rules set by Registries (For example, Verisign is the registry for .com, .net). I would assume these rules go back to the original issuers of the domains.
Regardless, I think that any registrar would have to have some "we reserve the right to shut down your domains" clause given the number of botnets and other uncool stuff on the net which use lots of domains (see http://en.wikipedia.org/wiki/Conficker#Payload_propagation for example)
From a pricing perspective, I think your market might be limited in that companies needing security would probably purchase a separate solution and not use Dropbox.
But you might be able to get a revenue stream from prosumers / tech consumers. And your costs wouldn't be high since you ride on top of the dropbox service. And I could be wrong about the market of course. ;-)
Sorry to hear it Chad. I haven't used Notifo but followed your work, and saw you present a few months back at the real-time API meetup at Mulesoft's offices.
You seem like a sharp guy with tons of enthusiasm for what you have built (and will build in the future), so I am sure you will be successful here in SV.
On the single co-founder aspect - wow, I can't imagine. Aside from the moral support though, one thing you might want to consider is how a second cofounder could provide complementary/different skills from your own.
For example, partner with a business type person. Business models are hard, but lots of people have been through the experience of testing various pricing models, and could probably help you get closer to $$ faster.