This will give you false positives when you input characters not allowed by Twitter. I think they only allow [a-z][_][1-9].
Great point. I'll implement that once I at back. Was originally just thinking about simple names, as this is just a personal tool. Thanks for the feedback!
Sorry about the initial posting - it was working locally, but not on heroku. It's been fixed now - thanks for the feedback!
How does this get around the Twitter API call limit?
The web interface isn't restricted. I suspect this is just checking http://twitter.com/foobar to see if the user foobar is in use.