What is your Personas version and support package? Do you know if you have the latest update to the Personas Silverlight client?
Just asking because I remember that in an older Personas release the radio button didn't work correctly but I just tested it in Personas 2.0 SP1 and the latest Silverlight client and it works fine for me:
This is how the radio buttons are defined:
The script behind "Radiobutton":
In step 1, I'm copying the selected value which gets written into the original text field that was converted to radio buttons. The original field is hidden automatically. Then the selected radiobutton value will be displayed in a field on the next screen:
Is this something like what you're after?