I've been doing some reading up on web forms lately as we're working on introducing a new feature to the University's content management system. The forms that will be generated obviously need to be accessible, but we're keen to help publishers ensure they're as usable as possible too.
It goes without saying that my number one point of reference has been Caroline Jarrett and Gerry Gaffney's book - Forms that Work. The accompanying website has tons of useful advice and links to further information too.
Forms that Work - free online resources and book promotion
Caroline Jarrett recently gave a presentation at the Usability Professionals Association conference.
Design tips for complex forms - presentation slides from Caroline Jarrett
A great introduction to making web forms accessible is provided by Webcredible.
Making accessible forms - article by Trenton Moss for Webcredible.com
Jakob Nielsen looks at the things you should consider when using checkboxes and radio buttons.
Checkboxes vs. Radio Buttons - article by Jakob Nielsen
A more techie article, but interesting and useful nonetheless. I like the breakdown of the guidance here into what's accessibility, what's usability and what's law.
Finally, a great article from Web AIM (Accessibility In Mind):
Creating Accessible Forms - WebAIM.org