Not to throw any more wood on the fire, but I can very much remember
pressing 2 buttons at once just so that no buttons were selected.  I
never really liked playing by the rules.  Which I guess is the reason
that I say it is 100% okay to have no default selected for radio
buttons.  There is definately a time when you want an explicit action. 
Having tested multiple consumer facing e-commerce related websites, I
have a rule of thumb that I take into testing the first round.  "If it
deals with money, defaults generally aren't good."  There are all sorts
of different reasons, for all sorts of different things.  In the end,
run it in front of some users and find out what they think.

-Bryan

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