In 2004, Harrah's bought Binions Horseshoe Las Vegas along with all rights to The World Series of Poker (Tourament, TM & Web names) for approx. $50 Million, much of that in outstanding Horseshoe liabilities. The domain name itself, I believe, was part of that deal.
Interesting to note is a massive ongoing lawsuit over WSOP.com that is independently owned - One can read the present domain owners position on his website.
FYI - Harrah's owns many of the typos and will pursue legally those who they feel infringe.