Video: Trademark law can be a tricky thing, but people normally have a good idea when they're illegally using a likeness. Wait, you do know that filming the Hollywood sign without permission can get you sued, right?
As Tom Scott explains, trademark rights don't expire as long as the property is in use. That means that if you film the nearly-100-year-old LA landmark for any commercial use, you need to pay licensing fees to the Hollywood Chamber of Commerce. If you don't have the time or money to either pay up (or make sure you're not in violation), better be good with that pixellating tool.