We just released a new plugin that you might find useful. It allows the blog owner to easily create modal popup windows with just about any content including wp shortcodes.
It works out of the box with Contact-Form-7 Custom Forms using Ajax validation and submission.
Modal windows are a great marketing strategy and our ideas for future updates will allow for multiple modals as well as allowing to load the window on page load.
Easy Modal on WordPress.org
Easy Modal Plugin Page - Info, Demo & Discussion.