I do think the problem is lag compensation, but I do not believe that people are cheating this way. Lag compensation is when your average network lag is added to your clock, so what the opponent of the player is seeing is that the player takes a long time about moving, however he's actually just experiencing lag and gaining time due to lag compensation.
I was accused of cheating, because I have a board set up. I take time not only to make my move, but write down each move. This eat's up almost ten seconds and is why I like to add the seconds to the time I have and play games I have time on. Someone not doing this has extra time on me, of course. I felt I was being accused by a sore loser and couldn't believe he would accuse me. It seemed, well, childish.
There is cheating software, I have it on video. I was blocked from playing when my opponent had 2 sec on the clock and I was in winning position. The same happened earlier on, also in long time games.
Sigh... why can't players use a 1-second increment and not worry about trying to out-premove their opponents?
