Bing does have a similar service, it's called Microsoft AdCenter and it now include the Yahoo also. If you advertise on Bing/Yahoo your ads will show on both search engines. The CPC is lower too. The partnership will not be complete until the end of October. But I have ads on Yahoo now and its showing on Bing too. I have seen a 4/1 ROI on my investment. It's a good alternative right now. I don't know how it will pan out in the future, but right now it's worth it.
I have to disagree with what someone said above, I'm not sure how having adwords listed in the engines brings credibility, in fact a higher percent click on natural listings than on adwords. As for the marketing piece email marketing, seo, articles, press releases, adwords, the whole schabang. I'll focus in on the ones that are more productive and exploit those.
I think it is worth to work hard to get good ranking only with homemade SEO. I started my website 1 month ago. I am using adwords to have some traffic. It cost me about $1.50 per click. I have around 50 visits per day and one sale every three or four days. Not good at all. I know that if i crank up my page rank I can have easily 1000 visits per day. Which would be great.
Organic ranking is definitely important, make no mistake there, conversions are much higher on organic than on AdWords. I just believe a combination will give someone a higher ROI, if done correctly. Im sure there are countless examples of people losing thousands of dollars on AdWords losing lots of money. These stories are common. The trick is knowing all the querks of doing AdWords, getting certified and realizing that in my cases AdWords will lie to you to get you to spend and lose more money. Reading between the lines is essential when reading the Google Adwords learning center stuff. Perhaps i just got off on a tangent.
As far as I know, there's been nothing ever published about this. If you have a link or document, would be much appreciated. I have some data from a few years ago that showed as many people click the natural listing as did the Adwords for one client for the same keyword when they both showed high (a few in first position). This was one client so it does make it a trend but based on that, I'd say it's close to 50/50. Of course, these were good ads, the organic may not have been so good so there are other variables to take into account. Good point about using the methods which are more productive, more profitable. SEO is important and should always be considered. But not at the expense of forgetting everything else. Good marketing means using more than one strategy.