You gotta be kidding. Vegas is a big expensive trip, even for US players. Why would any of the players around the earth go to Vegas when they do the same for free at Full Tilt or Poker Stars, while just playing in their spare few hours each day? These sites have virtually no overhead by comparison, and probably thought wisely enough to set up where the taxes (if any) are negligible. Are the 2 biggies both out of the Isle of Man, or is that just PS?
Just a quick glimpse of how many tables FT & PS have going at any given time astronomically eclipse the number of players at any casino poker room, okay slot machines included. To boot, the casinos just don't offer the stakes at any given game online. Perhaps Bobby's Room (at the Bellagio???) might be an exception. Poker Orifice's numbers do not surprise me in the least.
But really, I saw Antonius playing Hansen for 10s of thousands per hand on FT - this was even while WSOP was going on last year, and they were both (finally) in Vegas.