Flex 3: Panel is not showing rounded corners at the bottom even after setting cornerRadius

You will get rounded corners at the bottom only if your panel contains a control bar. So if you need rounded corners at the bottom of the panel then add a control bar to your panel.

< mx:Panel >
< !-- What ever you already have -- >
< mx:ControlBar/ >
< mx:Panel >

Here the colour of the control bar will be the colour of your panel border.