After looking around, your plugin is the best I have found for Mailchimp subscription.
I am having some trouble customizing the look of the form though. Ideally would like to have the subscription form be a single line. Would love to not have a title and just have the text inside the box (which your widget does) and then would need to have the submit button on the same line as the email field. Can you advise of the way to do this, ideally without hacking the php files (do not want to loose the hacks upon upgrades).
I was able to customize the size of the email field in CSS, but long term it would be great to have that as an option in plugin settings
You can see the subscribe form in action on this page in the header widget area.