Possible Duplicates:
Is it reasonable to assume my visitors have javascript enabled?
How many people disable JavaScript?
I know that JS should only be used to enhance functionality and users with JS disabled should still be able to use your website. But how often does that really happen nowadays? A majority of modern "Web 2.0" pages need JS for added functionality.
In other words, how safe is it to assume that most of your users will have JS enabled?
Does anyone have concrete data on this?
You may like to review Web Content Accessibility Guidelines 1.0. (Edit: It's worth noting that this link isn't about what you asked specifically, it's about acccessibility, which is related).
My opinion on the matter is that it is not so hard to write your site in the fashion of 'Graceful Degradation' or other variants of the same theme. The underlying strategy being that you write your site that it works for both (and in one model you write it so the JavaScript works first, then implement the 'non-JS' mode, and in the other vice versa).
It's really not so tough, and is generally helpful to everbody.
Practically all of the PC/desktop/laptop users have JS enabled. Your current major concern should be the mobile market. This is growing very fast. More than often JS is (by default!) disabled to improve browsing performance. This is however slowly but surely getting a better coverage. Then there are screenreaders and textbased browsers and of course searchbots. Practically none of them supports/uses JS. With regard to searchbots, this can however change in the future.
All with all, you should try to use JS only for progressive enhancements. The website should be designed the way that it provides and maintains the same functionality with JS disabled. The best way to achieve this would be building the website without any line of JS code and when you got all the minimum required functionality to function, then add JS for the enhancements like ajax, eye-candiness, user experience and so on.
Noted should be that this may of course depend on the target audience of the website in question. Is it a news site? Social networking site? Intranet site? Discussion forum? Etcetera. Stackoverflow for example is unusable on my Blackberry with JS disabled. I can't even login. It is however gracefully displaying a warning bar in top.
How safe is it to assume that most of your users will have JS enabled?
That depends on who your users are! Who's your target audience?
Does anyone have concrete data on this?
Sure, it's easy to track, but the data is only meaningful in the context of that particular site's audience. A site selling vacation rentals will have totally different users than a site for fantasy football, for example.
The short answer to your question, though, is that the only people that disable it will have done so intentionally, so they know how to put it back. It would probably be better to focus on cross-browser issues, users of mobile devices, maybe impaired users (depending again on your target audience) before worrying too much about whether they've disabled javascript.