I don't know if this has already made it to every single cat lover in the world already, but in case it hasn't:
An interesting discovery from someplace called the Monell Chemical Senses Center, which spends all its collective time working on science related to the senses of smell and taste and on chemical irritation.
Thanks to a genetic mutation somewhere back in the mists of time, cats apparently have lost the ability to taste sweetness. Their tongues simply don't include the sweet receptors that humans and other mammals have.
Here's the press release on it. Or, for a more general approach that talks less about base pairs and start and stop points on amino acid coding sequences, here's the CNN take on it.
Short form: It's not so much that cats just don't have sweet receptors, more that they don't work. Mammalian sweet taste receptors are created by a combination of two proteins, called T1R2 and T1R3. Thanks to the mutation, the gene sequence that defines T1R2 isn't read properly, and the protein isn't made. T1R3 IS made, but without T1R2, it just sits there and doesn't do anything. Result: cats can't taste sweet.
Cat owners have guessed this for a long time - cats don't care about sweet things the way dogs and other animals do - but now we know why.