Joshua Porter's "Designing for Sign Up" presentation from Webstock, 2009, does a great job of focusing the task of sign up on user motivation and anxiety, and thinking through scenarios that help remove barriers to entry. As he says, the form itself is the least of your concerns. Porter describes the entire experience leading up to the call to action, providing excellent examples that ease users into sign up.

http://www.slideshare.net/bokardo/designing-for-sign-up