It's called
Product Placement. Companies are known for for paying to have their products in movies, commericals, television shows, print advertisements, etc. Apple is one of the more successful companies utlizing product placement. Apple has such distinctive features (the white iBook, the Titanium PB, the look of the OS) that when you see it feature you immediately know what it is.
I read an article recently where Dell is trying to hone in on this market. Dell's wasn't a popular choice because they refused to allow the logo on their machines to be reversed. With the logo reversed the viewer would be able to read it from left to right instead of backards.
Apple didn't have the same problem as Dell becuase the apple silhouetteis distinctive no matter how is placed.