Wow! Great work. If I had a request, it would be to support more exotic TLDs and other registrars--but it's amazingly useful as-is. The price comparison is killer; I had no idea I could register a .TV domain for 10 bucks. I'll definitely be using this.

Is your VPS doing the DNS lookup, or is that client side? If it's done on your server, I would suggest that the site preview feature could be made click-activation rather than automatic. It's already pretty slow to load, and I would be just as happy with a link to open an unavailable domain in a new window. Basically, if the preview slows down the domain availability lookup, I think it should be pushed to a lower priority.

Overall, very slick! Congrats on a successful side-project.