A month or so ago I looked at the stats for Just-Dice.com. There were about twice as many losers as winners, and the house edge there is about half that of SatoshiDice at just 1% compared to SD's 1.9%, so I imagine the ratio of winners to losers is even lower at SD.
Hmm, I think you are right. The more someone plays, the closer they get to the expected value. That will narrow the distribution, putting more people on the losing side.