A new radio button type would be similar to the existing menu type, but rather than displaying a <select> form field, it would output a radio button group. In Drupal CCK radio buttons are actually an alternative form of the select type. So there are two possibilities for us: 1) extend the existing menu type to offer a radio button display or 2) add an entirely new type. The latter is probably simpler for people to understand - in my first experience with Drupal I thought it didn't do radio buttons!!
OU designers would prefer radio buttons to be offered horizontally rather than the default vertical. This can be achieved by extending the "felement fradio" class automatically assigned to these fields. It would allow more flexibility to add a horizontal or vertical option to the custom field definition form and depending on the setting chosen, add an extra "horizontal" class, which would be defined in the standard stylesheets to display inline.
I have a number of requests for this (for our Sept 2011 release) and am more than happy to program myself and offer a patch. If whoever triages this thinks that's OK, please assign to me and I will add to the OU development schedule over the next few months.