Wired has an interview with Stan Lee about his cameos in Marvel movies, the most recent of which is his appearance in Iron Man. In the movie, Tony Stark only sees Lee's character, surrounded by beautiful women, from behind while on the red carpet, and says hi to "Hef," quickly moving on before Lee turns around. It creates an interesting ambiguity, leaving the audience to decide if Lee is playing Hugh Hefner or an alternate-universe Stan Lee who has a lifestyle very similar to Hugh Hefner. Or, most unlikely, an alternate-universe Stan Lee who spends much of his free time emulating the lifestyle of Hugh Hefner, renting smoking jackets and hiring expensive call girls to accompany him to premieres and parties.
While we may never get closure on that point, Marvel's announcement of an Iron Man sequel to be released in 2010, as well as three other Avengers movies to take place in the same shared movie universe as Iron Man, points towards plenty of future cameo opportunities for Lee.