A role in the film is NOT a cameo.

A cameo is when a notable figure shows up for one scene in the film just to show their face. John McCain in Wedding Crashers comes to mind. If you have not seen that film (which you should), John McCain shows up at the wedding of a powerful family as a means of portraying the family's power. The film would have worked just fine without him. A cameo is NOT a notable figure who has an impact on the story and moves the plot forward. If you were to look at Wedding Crashers again, Christopher Walken was not a cameo.

Matthew McConaughey in Tropic Thunder is NOT a cameo. I am really getting annoyed with reviews that refer to his role as a “cameo.” For those of you that don’t know, Owen Wilson was originally set to play the role of Rick Peck, but dropped out after his suicide attempt. If Owen Wilson had been in the movie, no one would have referred to his role as a cameo just like they didn’t in Night at the Museum.

For some reason, Matthew McConaughey is not taken seriously anymore which is why I think people are calling it a "cameo." I understand that he's made some poor choices in picking roles, but the dude can still act. With every film he does, he brings a certain personae that can work for the film or against it, but either way, he has a presence that can't be denied. It's just sad that now-a-days instead of being respected (have you SEEN Dazed and Confused?) he's considered a joke.