I found a conflict with MailChimp's official plugin: http://wordpress.org/extend/plugins/mailchimp/ when it was set to include the JQ Datepicker UI.
Originally I thought it was the MailChimp plugins fault, but it looks like they're enqueuing datapicker.js and whereas in GF's form_display.php and export.php files it doesn't look like it's getting enqueued. I'm obviously not a php/js expert so I'm not saying anything for certain. Just hoping you can look a at.
(yes I could build the signup form in GF, but that's not the point...)