Perhaps the right question is: "when will browser support for CSS 3 become good enough?"

That's hard to say. Browsers such as Safari support quite a lot of CSS 3 (such as multiple backgrounds). You can see demos at CSS 3 info.

Unfortunately, IE6 is still the dominant browser. Until that changes, using CSS 3 will be limited to minor enhancements. If you do anything too dramatic with it, IE will suffer. We also have to hope that IE7 will be updated more efficiently than IE6 was.