Website Optimizer by Google AdWords is a multivariate testing tool, that is to say, it takes A/B testing one step further. Instead of testing 2 versions of a page, you can test multiple page elements and the various combinations.
I participated in the beta test of this and my initial concern was that it may not be search engine friendly due to the changing page elements, however, after a short call with the Website Optimizer Product Manager, he confirmed that it would not have any impact on organic rankings.
If you are still concerned about it, then you can always set up a specific landing page that is not linked to from your main navigation and use either the robots.txt or meta noindex tags to prevent search engines from crawling those pages.
Once your AdWords account is fairly well optimized, I would highly recommend trying out this tool. You will learn new things about your website, its traffic and motivators. Just make sure you carefully plan the test elements and don’t test too many elements at once to ensure that you can run through enough iterations with conversions to gain meaningful data.