In their study specifically of Bwin (an online casino) they found that "of the 4,222 casino customers, just 2.8% — or 119 big losers—provided half of the casino's take, and 10.7% provided 80% of the take".
However, online casinos don't attract the kinds of customers or the level of action that many physical casinos do. Many older players - who provide the vast majority of slot machine revenues - won't go near online casinos. Further down in this article they quote a marketing consultant for dozens of physical casinos saying "Bwin results are in line with his own estimates, based on confidential casino data, that many U.S. casinos get about 90% of their revenue from 10% of customers."
I have read a few academic papers on the subject, that also indicate numbers from 80% up to 92% being provided by roughly 10% of players. The fact that in the Bwin study, 2.8% provided 50% of the take, is breathtaking as well.
