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.
Have a look at when can I use for support tables.