I know that cheating on internet sites is a controversial topic and that lots of keystrokes have been squandered discussing it.  What I haven't seen though is much discussion of opening-specific cheating.  For example I just experienced the following:

I was Black and against the Center Game quickly rattled off the following, and White was moving equally as quickly, as you can see from the timestamp with each move in the pgn:

1. e4 {0:15:01} e5 {0:15:03} 
2. d4 {0:15:01} exd4 {0:15:07} 
3. Qxd4 {0:15:04} Nc6 {0:15:10} 
4. Qe3 {0:15:08} Nf6 {0:15:14} 
5. Nc3 {0:15:12} Bb4 {0:15:18} 
6. Bd2 {0:15:16} O-O {0:15:22} 
7. O-O-O {0:15:19} Re8 {0:15:22} 
8. Qg3 {0:15:23} Rxe4 {0:15:24}

Now my opponent, rated low-expert, in this critical position, actually had to think for a minute or so about his response, and then came up with the book move: 9. a3.  If you play this system as White, either you know the book move or not.  

A minute on the other hand is certainly enough time to look this up in chessbase though.  Not absolute proof by any means, but just certainly suspicious in my mind.  Anyway, it just improves my game, I go look stuff up in one of my books or in chessbase after the game - doing so during the game isn't going to help me prepare for the World Open this summer.