I once read that for every question you add to a form you lose 40% of the people viewing it.
Putting it into action- currently working on a web development firm with leads gathered via AdWords, and trying to optimize the deal pipeline. At first there was a "Click to Contact" button that then forwarded to a Wufoo form- just email, website, (optional) phone number, and a comment.
I read that stat, and put up a "Quick Contact" form (also Wufoo- they rock :) ). It just had email, and website, and said "Plug in your email and website and we'll respond with a full proposal for the project." (We only provide one service so it's pretty standardized.). Whereas before we were getting about one lead every other day, we started getting 1.5 a day, an increase of 200%.
We then removed phone number and website from the contact form, since the majority of respondents put that in the actual contact form- immediately, we went from 1.5 to 2 a day. When dealing with leads for development gigs at a couple thousand a pop, that extra .5 helps a lot.
Anecdotal, evidence ahoy, but hopefully that helps some.