By representing a single form as a funnel, you can isolate drop-off to specific form fields, but how can you track individual fields when each funnel step in Google Analytics corresponds to a separate pageview?
The answer is virtual pageviews.
Without loading a new page into the browser, you can take advantage of the form’s built-in event handlers, such as onBlur, to generate a “virtual” pageview as the user completes fields and clicks the submit button.
These virtual pageviews will match any corresponding goal URL or funnel step that you have defined, just as an actual pageview would.
<input type="text" onBlur="if(this.value != '')_gaq.push(['_trackPageview','field-completed-phone'])" />
Excerpted from the Google Analytics Conversion Funnel Survival Guide.
Thanks to Brian Clifton for demonstrating this technique.