Hello dlewis,
If you examine RadFormDecorator's CSS in the skin file, you will notice a rfdRoundedCorners selector. You could change the value of the radios, but this will not work in IE and Opera that do not support border radius in CSS. Unfortunately at this point there is no solution that will work in all browsers because for IE and Opera we are outputting extra code and the values there cannot be changed.
Greetings,
Georgi Tunev
the Telerik team