Luke Wroblewski's book on form design is finally out. From the author:
Forms make or break the most crucial online interactions: checkout (commerce), registration (community), data input (participation and sharing), and any task requiring information entry. In Web Form Design, Luke Wroblewski draws on original research, his considerable experience at Yahoo! and eBay, and the perspectives of many of the field's leading designers to show you everything you need to know about designing effective and engaging Web forms.
More info and purchase options available from the book site.