DD_roundies is another rounded corners solution that is very easy to implement & cross-browser compliant.
This rounded corners script doesn’t use any images, works in IE6 without AlphaImageLoader & lightweight (3.62kb compressed).
Radius of the corners are not fixed & can be defined.
As the major problem for rounded corners lies beneath IE, this rounded corners script uses VML (inspired from Jonathan Snook’s solution). A possible future problem is IE8 -currently- does not support VML.