The past few weeks have seen a rise of in-game casinos on some servers. I haven't noticed any on my home server of Shadow Council, but on Arygos, where I've been toying with a horde hunter (level 34 right now), I can't walk two steps in Orgrimmar without seeing a /yell advertising a chance to win big.
As of April 20th, however, this will now be prohibited. In Blizzard's words, after that date, "any advertising for a casino is not permitted," and players are encouraged to report advertisers to the GMs. I have to say, I'm not sad to see the casinos go. It's a clever way to make money, yes, but I don't like seeing players advertise repetitively, and of course, the house always wins. It's nice that Blizzard was so responsive about this issue.
While the new rule doesn't prohibit casinos directly, I imagine it will put a stop to most of them, unless there is some secret network of private casinos out there, with transactions carried on in mysterious raid and party chats, or custom channels. If there is, carry on - that's super-cool.