A good bet might be poker tournaments. A quick check and I see the Taj Mahal held a Texas Hold'em tournament on Oct 7th, 2006.
http://www.cardplayer.com/poker-tournaments/764-2006-u-s-poker-championship
Immediately made me think of Joran Van Der Sloot.
Timeline fits - it's in between the disappearance of Natalee Holloway in 2005 and the murder of Stephany Flores in Lima, Peru in 2010 during a Latin American Poker Tour event.
He was also linked to the disappearance of two women in Bogota, Colombia who were known to frequent casinos, although police later ruled him out.
Pure speculation and a massive long shot, but I wonder if he ever visited Atlantic City on his travels.