Is there an API (possibly provided by Twitter, but not required) that will give me a Twitter username for every email address I give it?
Twitter's official answer on this is here - http://code.google.com/p/twitter-api/issues/detail?id=353
“This API method is currently being offered in closed beta and we are not accepting new partners. However, we are working on a long-term strategy for this method and will remember when we decide to open it again. I apologize for the inconvenience this may cause. same time, and please let me know if you have any other questions.
As far as I know, the Twitter API does not disclose or otherwise prevent access to the email address.
I understand your needs, as I would also like this to be available, but I believe this is the solution Twitter has come with.