Professor Wikipedia says
"Theobromine is much less toxic to rats and mice, due to their relative genetic similarity to primates"

Actually this stuff is pretty interesting - cats are very sensitive to theobromine, more so even than dogs, but they have no taste for sweet foods so they're less likely to eat it. Dogs are slightly more resilient, but enjoy the taste of chocolate, which makes it far more dangerous to them. Humans can eat a lot more than dogs, but in large quantities it can be toxic even to us. Rats are actually the best - a rat can eat 25% more chocolate (in proportion to its body mass,) than a human before risking theobromine poisoning.

THESE ARE SOME THINGS I LEARNED ON THE INTERNET JUST NOW
( Zoroastermouse thinks therefore he am, Sat 24 Dec 2011, 1:57, 1 reply)

I call bollocks.
Cats certainly do have a taste for sweet foods; I have to supervise mine all the time whenever I have chocolate around. Chocolate, or croissants, or butter pastries, or anything similar susceptible to kill them from cardiac overstimulation. If anything I'd say that dogs were less discerning, from having known a dog who mistook a giant rock for his chew ball and lost two teeth as a result.
( .Yeti., Sat 24 Dec 2011, 2:11, closed)

Hmm
That's interesting; apparently cats lack the full set of genes required to build the taste receptors that bind to sugar proteins, but the internet seems full of anecdotal accounts of cats liking sweet foods. Apparently cats have all sorts of receptors for other proteins that humans don't have, so maybe there's something in those foods that cats like apart from the sweetness?

I am not a scientist.
( Zoroastermouse thinks therefore he am, Sat 24 Dec 2011, 2:42, closed)

it could be the fat content
cats love fat, it being present in all their meaty meals. Cats famously love milk and cream, which could also be because of the fat content. Standard chocolate (e.g. cadburys and equivilants) has a hell of a lot of fat in it, so it could be this they're going for rather than the sugar.
( ferret -> twitter.com/syfyautocorrect <-, Sat 24 Dec 2011, 3:35, closed)

My old cat used to go nuts for CHOCOLATE CAKE!
Proximity to it turned him into a cake-eating zombie, who would just blindly walk/climb/jump towards the cake, unrelentingly.

If I had a plate with a cake on it, and held the plate in the air so he couldn't get at it, he'd attempt to climb up me.

The only course of action was to attempt to eat ALL the cake in one go, but then he'd just try and climb into my face to access the cake I was chewing.

Finally, you'd have to dispose of the packaging in a proper dustbin, because he'd knock over any kitchen bin or waste paper basket to eat the wrapper.
(eViLegion Chief Commissioner of the Scottish Lunacy Board, Sat 24 Dec 2011, 12:49, closed)

Yeah, I'd go for lipid and protein content.
Chocolate always has some kind of lipid and cakes and the like have protein in the shape of egg whites and often have lipids also.
As a kid we had a cat who would beg for peanuts and crisps and I put that down to the salt and fat. Oh, yes, another thing cats love is salt and I'm sure I read their food is much saltier than dog food which is why you can feed a cat dog food (plus other stuff for nutrients) but shouldn't feed a dog cat food.
Oh, as for Rats I think they're evolving so that they can eat almost anything including (relatively) large amounts of cyanide and arsenic.
(Me, I'm not... in any way intoxicated, Sat 24 Dec 2011, 16:50, closed)