I think that depends on the type of web application you are going to build. For example in an e-commerce application the checkout process should propably work without java script because there are some people who deactivate js for checking out (in our experience). In a web 2.0 application in my opinion it isn't necessary to support non-js browser experience.
Developing for both also complicates the development process and is more cost intensive. you have double your web test efforts (testing with and without js) and also think different in the planning phase.
