(2006) Label Placement in Forms - Caroline's Corner article for usabilitynews.com
(2010) Label Placement in Austrian Forms, with Some Lessons for English Forms - article by Caroline Jarrett for usabilitynews.com
(2010) Label Placement in Forms presentation on slideshare by Caroline Jarrett
Forms that Work: book website, plus additional resources and articles
Carolines basic advice:
1. If the form asks only for simple data* and is short, opt for right-aligned labels placed to the left of the field or left-aligned labels placed above the fields.
2. Left-aligned labels placed above the fields may be more convenient if your form needs to be translated, as the variable space can be helpful for allowing additional space for the expansion you'll get when you translate from English to many other languages.
3. If the form is long but only asks for simple data, opt for right-aligned labels placed to the left of the field.
4. If the form asks for anything other than simple data, or you're not sure whether users will consider the data to be simple or not, then opt for left-aligned labels placed to the left of the field.
* Simple data: Information that: the user has conveniently in their head, the user willing to put into the form, and can be requested using a label of one or two words.