After upgrading to build 3759 I get "Live Preview Error" error message when trying to launch the live preview. It was working just fine before the upgrade. The content of the error message states "Open an HTML file or make sure there is an index.html file in your project in order to launch live preview."
Not only is there an index.html file in the project, I have it open in the develop tab when attempting to do a live preview. The simulate is working, just not the live preview.
Is your index.html file located in a www folder inside your project? I just tried it on a sample project and it worked fine. What happens if you try it on the "hello cordova" sample? Just create a new project based on that project and try it.