Google launched a new service for AdWords users: Website Optimizer. Google makes it easy to experiment with different combinations of landing pages and measure the performance using Google Analytics.
"The Website Optimizer allows you to test changes in the website content of your pages in order to determine what will be most effective in getting conversions. You choose what parts of a page you'd like to test - headline, image, promo text – and we'll run an experiment on a portion of your site traffic to determine which content on your site users respond to best."
Users will see different versions of your landing page and when Google has gathered enough data, you'll be able choose the best version.