External references:

Compatibility

Browser Status Notes
Firefox 3.6.8 Yes with caveats Needs -moz-border-radius, doesn't understand the top|bottom-left|right variants
Minefield (Firefox 4 beta) Yes with caveats Needs -moz-border-radius, doesn't understand the top|bottom-left|right variants
Iron Yes
Safari Yes
Opera Yes

Note that the radius doesn't care about the text, so you probably need to add some padding to the style as well

This should have rounded corners   This should have rounded corners at the top - doesn't work in Firefox 3.6.8 nor 4 beta   This should have rounded corners at the top - does work in Firefox 3.6.8 and 4 beta