If you want to get a sign on ESPN's College GameDay you have to make it through ESPN security. They screen the signs for religious and political messages and, more generally, anything that could be deemed offensive. One sign that fit the criteria for offensive—the "Pussy" sign with Jonathan Martin's face on it—made it on today's show. Here's how.
It's pretty ingenious, really. They apparently affixed a dummy sign—"Hi Lane"—to the real sign and when security saw yet another Lane Kiffin sign at USC, they likely just waved them right through. Then when they got near the set, they (eventually) Scooby-Doo'd the top sign, revealing the pussy sign. It probably all happened in a matter of seconds—you can see the same Obama sign right next to it—of the cameras panning to that section of the crowd.
It's just a shame these USC fans used up all their cleverness getting the sign on TV and not on the sign itself.