I am now running Tiger, but have used all versions of OS X (and Safari) and never have I had an issue accessing the Washington Post site, so I don't believe the problem is with the site.
As a wild guess, if you manually cleared your cache and cookies, you might try to again clear your cache and cookies, but this time using one of the third party cache cleaners such a Cocktail, Cache Out, or Onyx. Also, clearing your Safari History couldn't hurt.
Sorry my help is not much.