[scoot]

"Someone should build a (subscription) service where you can embed a bunch of fields (and labels) onto a form with a single line of Javascript.[...] The performance of the fields (their effect on conversion rates) could be measured continuously."

Not quite the same, but sounds a bit like inForm[1]. These guys presented at a recent HN London meetup[2] and this was the meetup.com description which summarizes it better than I could:

"Ever wondered how users engage with your site, where they get stuck or how long they spend making choices? Forward Technology's upcoming form analytics service inForm can help you answer all these questions and more. Without any configuration Inform allows you to quickly build up a strong picture of what happens when real visitors interact with your forms."

Beta sign-up for HNers here[3]

[1]http://inform.forwardtechnology.co.uk [2]http://vimeo.com/32617520 [3]http://inform.forwardtechnology.co.uk/users/sign_up