The way to customize the look & feel of RadRadioButton is to choose one of the ready-to-use themes for RadControls and make your customizations upon it. Currently the Vista theme is the one that is closest to the appearance of MS controls. You can copy the style and brushes for RadRadioButton and alter them to be similar to the MS native RadioButton control.
You can get the style for RadRadioButton for Vista theme from your RadControls installation folder. You will find there a Themes folder, which contains resources for all themes. Drill down to the Vista theme and copy resources related to RadRadioButton from Button.xaml reource dictionary. If you have troubles finding or copying the needed resources, you can request them from the Telerik team as well.
Although we do have plans for developing a theme that serves the exact same purpose of RadControls being visually consistent with MS controls, at this point we cannot engage with a certain deadline.
the Telerik team
Register for the Q2 2011 What's New Webinar Week. Mark your calendar for the week starting July 18th and book your seat for a walk through of all the exciting stuff we will ship with the new release!