I'm at WebVisions 2007 where Aaron Gustafson just gave a great talk outlining many of the building blocks required to develop effective forms with a strong focus on valid and efficient XHTML and CSS code. I've posted my notes at Functioning Form for those interested in the development side of Web Form Best Practices:

WebVisions 2007: Learning to Love Forms

Aaron is a member of the Web Standards Project (WaSP) and the Guild of Accessible Web Designers (GAWDS). He also serves as Technical Editor for A List Apart, is a contributing writer for Digital Web Magazine. In his WebVisions 2007 session, he explored forms from top to bottom, examining how they work and how their components can be incorporated with other elements to maximize accessibility, improve semantics, and allow for more flexible styling.