So I've been doing a bit of research on CSS3 and all the fun stuff you can do with it and it got me thinking about the wide scale use of it. So when should we start using CSS3? I'm not sure as to if all the current browsers support it fully or if it is even a completed language yet. I've seen several previews on css3info which makes me think it's not ready yet.
Most of it is not ready yet or even fully formulated. Some browsers have begun to implement little bits of it, like border-radius (either via vendor-specific prefixes or by using border-radius itself). So you can use it in little doses, but most of it will not work in a lot of browsers. IE9 has started to support little bits of it, but not any previous versions.
There are some scripts out there that can help older browsers imitate CSS3 features, but it's a bit dubious to be loading up sites with JS just for that.
With CSS3 you can use the things that will have no impact on your content if they don't work or get changed later. Things like Ralph mentioned such as border radius, box shadow, text-shadow are just decoration and if they are missing the page still works fine.
If you wait for all css3 to be finished the you have a long wait as it probably never will be but what is happening is that small sections are becoming main-stream and working in modern browsers and you can use these quite safely as long as you plan for the worst.
If you are developing for the iphone perhaps then you can use Safari's advanced features quite safely knowing that only Safari needs to be supported.