I checked this out, I don't seem to have the problem on my version, other then when using small number for the curve (5,8,10, probably anything below 15) just looks terrible, not only on the stage, but also after export... it's not smooth, you can see the joints of the curves with the lines, but they are curved correctly.
I don't think it is limited to CS5, it is probably and has probably been prevalent from the early days of Flash. It may have to do with the rendering capabilities, and possibly limitations of the monitor in combination with that. I am pretty sure if you zoom in you will see a more perfect line, but thin lines in Flash that are not straight and vertically/horizontally oriented will usually display scratchy.
Here are some findings since my post. Flash CS4 brings in the rounded rectangle perfect. Using a new CS5 Flash movie the rounded rectangle does not work as it should. Saving a CS4 Flash fla into CS5 causes the rectangles to draw perfect with rounded corners. In the past I have had no problems drawing my rounded corners even with a new CS5 title. The one change since I noticed is that I have a new GTX 470 graphics card. But still in CS4 it works but not in CS5 on new titles.
A bit puzzleing.