Jimmy,
I have the same problem with wanting a checkbox to be unchecked. I had created a checkbox using FB and left the Validation option unchecked since I didn't want that feature. So, I followed your advice to manually remove {echo "checked=\"checked\"";} associated with my checkbox; so, that worked.
But even after doing that, there must be some validation artifact there (even though I didn't select any Validation when I created the checkbox) that is requiring that the field to be checked. If I don't check the checkbox, I see a very narrow red box where an error message would probably be. It only goes away and allows the Submit button to work after checking the checkbox, i.e., I can't Submit the form if the checkbox is unchecked.
I created a simple checkbox that can be seen at test5.php so that you can see what I am talking about.
Any advice on how I can manually correct this problem? Thanks in advance.