It's true...when the scene was filmed, the poster was for Gnomeo and Juliet. That movie was made by Touchstone which is owned by Disney so they could have just left it. But Cars was more popular and makes more money in merchandising so...
I'm sure we'd all be far more annoyed if that terrible movie was referenced. Luckily, Disney pretends that Gnome thing doesn't exist for the most part.
But I'm far more annoyed by the Disney Pop star that the studio just about had to force into the film. Luckily, they wrote a backhanded line for her "My Agent told me I should be here." OUCH!
But quite honestly? Disney ain't got NOTHING on NBC/Universal. Go ahead... watch ANY Universal produced movie recently. They have NOTHING but cross promotions with NBC in their films. Get Him To The Greek, a brilliantly funny movie, has them appearing on The Today Show, a record producer who's favorite show is "Biggest Loser," among other things... not to mention even Despicable Me isn't even safe... filled with voice actors who are employees of NBC sitcoms... and the fact that Gru even stole the Jumbotron from Time Square that visibly has an NBC logo on it. i think that's the only reason he stole it. At this point, I'd be surprised if there wasn't a deleted sequence in The Lorax where they tell the Onceler of all the things he needs to appreciate "Like Whitney at 8:00 and the Chelsea Handler show at 8:30 on Wednesdays!"
One Cars 2 billboard is nothing.