Stan Lee will have his first appearance as himself in The Big Bang Theory S03E16 The Excelsior Acquisition.
At the beginning of this video we can see a little preview. Stan Lee opening his mansion’s door to Penny and Sheldon. He is dressed with a Fantastic 4 robe, in a Hugh Hefner style. Penny asks him if he is Stan Lee (obviously Penny doesn’t know this huge geek celebrity, which is a sacrilege).
Penny: Are you Stan Lee?
Stan Lee: Oh damn.
Penny: I am Penny and this is my friend Sheldon.
Sheldon: We’re not friends at the moment. It depends on how this goes.
After the clip, we can see an interview where Stan Lee one more time demonstrate how kind, funny and polite man that he is. He speaks about TBBT show and its creator Chuck Lorre, who is also his friend.
Stan Lee says that he will appear for a second time in The Big Bang Theory Season 4 
