I've written on the topic of usability/context-sensitive help in registration forms online before.

Another topic related to registrations is avoiding machine-automated registrations (preventing spammers from registering thousands of "garbage" yahoo.com and hotmail accounts, for example). The solution, obvious in retrospect, brilliantly devised by Dr. Udi Manber (now chief algorithms officer for Amazon.com(!)), was to introduce small puzzles that are easy for humans but hard for machines.

NY Times has an article on these so-called Captchas:

a collection of cognitive puzzles based on the challenging problems of artificial intelligence. The puzzles have the property that computers can generate and grade the tests even though they cannot pass them. The researchers decided to call their puzzles Captchas, an acronym for Completely Automated Public Turing Test to Tell Computers and Humans Apart

• Microsoft's Digital Pen technology - Mar 28, 2006
• Bring your outboard brain with you - May 11, 2005
• Testing applications 'for real' - Mar 08, 2005
• Too advanced Captchas? - Jan 21, 2005
• How to redirect after changing your domain name - Jan 11, 2005
• See more related posts in: Usability / Accessibility