Casinos have discouraged photography inside for ages - it's nothing new. This is usually under the guise of protecting their customers privacy.
Recently though, some have reported casino security taking a much harder line and in one case, a photographer was even detained.
VegasRex has been beating the drum on this issue for some time - if I recall, he even had a run-in himself. Some of his commentary is here: http://www.vegasrex.com/2009/03/22/photography-in-casinos-officials-respond/
The RJ is now looking at the issue as well, contacting Rex, the detained customer and a few security officials: http://www.lvrj.com/business/41647727.html
As someone who has taken literally thousands of photos of these places, inside and out, I have strong feelings on this. Now, I understand a private business can prohibit photography - I've been asked not to take photos - but the idea that this is some kind of security threat and that it could even possibly be a real offense is ludicrous.