Dude this is seriously cool. It's dead simple and the page is super clean. I think if you promote this properly it could be really popular. You could probably make some decent money putting up affiliate links for web hosting. I think you should seriously consider building a suite of webmaster tools that are similarly clean and simple. Also, you should consider using a much shorter (but descriptive and memorable) name.
Made this quick little tool today. Uses a host of proxies to avoid getting caught for spamming Google Toolbar PageRank servers; then we do a live ajax check towards Google as soon as the user finishes typing one of the 280 (or so) TLD domains.