It's tough because it's not as difficult as you might imagine to get your IP address rate limited. If a lot of people were using it at once and I was sending out hundreds of availability checking requests I'd probably hit the limits.

For example, I had this hosted on Heroku for a while and tons of requests were failing because Heroku uses shared IP addresses for all the sites it hosts. Evidently I wasn't the only person doing domain availability checks from their network.

Also, the UI would get pretty nuts right?