There is a good Google Blog post on the fact that you can now get Google Web Optimiser without using Adwords — just any Google account will do. They’ve also improved the workflow to make it easier and more effective to use.
Purple Motes points out that as your site gets larger and attracts more traffic the audience demographic may well change. Just using multivariate testing to optimise once may not be enough. The post has some good ideas on how to identify what it is really driving usage and what not.