Having spent several hours trying to update K2.js validation for radio butons I have to admit defeat today.

Scenario:
-extrafield radio selection
-user must make one choice only and it must not be selected on load

THe issues are:
-radio buttons - one option is selected by default when set to required or otherwise
-validation is really difficult in the k2.js code [well at least for me]

Is this addressed in SVN? : :(

haha its not midnight yet and I have beaten k2.js into submission B)

If anybody is having issues validating extrafields that are of the type radio buttons then try the attached k2.js.
It goes in 'media/k2/assets/js'

Its tested with 2.5.x and the file difference with 2.6..x seem negligible

Comments /changes are marked . I have added about 30 lines of jquery/javascript

Tested on IE8 /FF 19 / Chrome and up