The sign-up process is much more than just the form that gets a potential customer's details. The majority of the sign-up process actually takes place inside the customer's head. A customer will sign up - even using a crappy signup form as long as they're actually motivated to buy your product... not that you should make a crappy signup form - because that also just puts hurdles in the way of an otherwise motivated potential customer.
This awesome presentation explains how you can design your signup process to get customers motivated and rearing to go on your site. It points out where the friction is in the process, where they usually fall out of the sales funnel, and the ways to change your way of thinking about your site: to provide ample opportunities for a confused or skeptical customer to change their minds. It also picks out the pain-points for the sign-up process - giving some great examples of how to get out of their way and let them get on with paying you money straight away... always a good thing.
I only wish I'd seen the original talk, but the slides themselves speak the story clearly and are well worth a few minutes of paging through.