It is true that chocolate is not good for cats - but I don't know exactly how toxic it is to them. They react to the theobromine found in chocolate.

I do not allow my cats to eat chocolate, although I do know of some cats that do eat chocolate and are still alive

I think, in general, that cats love the smell of chocolate rather than the taste. Mine will both smell it, and even start drooling but will not actually eat it.

Check-out this link:
http://www.fanciers.com/cat-faqs/health-care.html

And this one provided by SpencerTheLion in a previous post:
http://amby.com/cat_site/plants.html