I don't think there is a default option- one can simply not select any answer.
But the point of the open text field for responses is that it provides input regarding what should be included on the next survey.
Can forms with radio buttons be submitted without selecting any? I hadn't noticed that. And anyway, if you select one by accident, in all browsers I remember using there's no easy way to unselect it without selecting another.
The first draft of the 2012 Less Wrong Census/Survey is complete (see 2011 here). I will link it below if you promise not to try to take the survey because it's not done yet and this is just an example!
2012 Less Wrong Census/Survey Draft
I want three things from you.
First, please critique this draft. Tell me if any questions are unclear, misleading, offensive, confusing, or stupid. Tell me if the survey is so unbearably long that you would never possibly take it. Tell me if anything needs to be rephrased.
Second, I am willing to include any question you want in the Super Extra Bonus Questions section, as long as it is not offensive, super-long-and-involved, or really dumb. Please post any questions you want there. Please be specific - not "Ask something about abortion" but give the exact question you want me to ask as well as all answer choices.
Try not to add more than five or so questions per person, unless you're sure yours are really interesting. Please also don't add any questions that aren't very easily sort-able by a computer program like SPSS unless you can commit to sorting the answers yourself.
Third, please suggest a decent, quick, and at least somewhat accurate Internet IQ test I can stick in a new section, Unreasonably Long Bonus Questions.
I will probably post the survey to Main and officially open it for responses sometime early next week.