We're mostly interested in the UK marketplace so this WebSideStory survey that has been linked from down below in MyWeb 2.0 for most of today should be taken with a pinch of salt. It does however highlight what we know from experience that volume isn't everything and search engines other than Google should be considered as important even if not primary parts of your search marketing campaign, becuase they can often convert a better % of visitors even though their overall traffic volume is lower.

From the survey:

Median Order Conversion Rate by Major Search Engine at Business to Consumer E-Commerce Sites, January 2006

For more info: http://www.websidestory.com/company/news-events/press-releases/view-release.html?id=319