Same problem. Very big issue for me, as I’m using it for a political campaign site I’m running for someone. Exactly the same as chooseamc; I can select the mail list after entering the API Key, but every registration attempt receives an error. I am running WP 3.5.1, and v 0.8.12 of the MailChimp Widget. I like the widget, but I can’t use it if I can’t trust it.