So it's not quite what you're asking for, in that you'd probably need to add som...

nialo on Feb 2, 2012 | parent | context | favorite | on: Beautiful Buttons for Twitter Bootstrappers

So it's not quite what you're asking for, in that you'd probably need to add something to automatically generate many color options or whatever, but http://www.mynaweb.com/ seems close and uses better math than A/B testing anyway.