I checked online and ran into this very interesting article. The author contacted the FAA's Eastern Region Office and asked for a formal legal definition:

"Reduced to basic terms, known icing conditions exist when visible moisture or high relative humidity combines with temperatures near or below freezing. Since clouds are a form of visible moisture, flying through clouds at an altitude that is near or below freezing would constitute flight into known icing conditions. The Federal Aviation Regulations do not allow for experimentation."

... "In explaining their definition of known ice, Loretta E. Alkalay, FAA Regional Counsel, referenced Administrator v. Curtis, NTSB Order No. EA-5154 (April 29, 2005). Here, the court ruled that conditions conducive to icing exists whenever near- or below-freezing temperatures and moisture exist together in a given area. It didn't matter that there were no reports or forecasts of icing conditions at any altitude anywhere near the route of flight."

So basically, if you take a non-certified aircraft into known icing conditions (temperatures near or below freezing, and visible moisture...clouds) you're in violation. The good news is that if you're reported, the FAA will use a matrix called the "Enforcement Decision Tool." Which forms a risk assessment of the incident and includes the pilot profile (was the action intentional, does the pilot participant in saftey programs like WINGS) etc.

What I would do, if you found yourself in known icing conditions, its probably best to fill out an ASRS Report. Also its great to participant regularly in WINGs as it does factor into the FAA's enforcement decision process.