I didn't hear the start of the conversation or the question but David was trying to say something and Simon constantly stopped him saying 'David, what stays in Vegas ...' Or something. And I'm sure Amanda said something like this is potentially career ending.

What was the question and what was David about to reveal?

Maybe Cowell did something illegal?

As David refers to Simon as 'My Simon' , the caller wanted to know what Simon's pet name for David was. Simon went very coy and told David not to reveal the name as they would probably all be taken off air for good. So we'll never know what the name was.
Sorry.

Judging by it all Im going to guess maybe the C bomb

Doubtful. You would be on it like a ton of bricks if he did.

That wasn't it. They were asking David what kind of things Simon writes on the notes he passes during the show.

Obviously they pass extremely rude comments about the acts. Comments on appearance and talent or lack of.

My guess is as the last poster said. Rude comments about the acts and the show.
The cynical in me thinks sometimes they give tips on what they should say about the act, what act to keep in if they depended on the judges votes etc