This article from Wired about why the body likes the taste of protein discusses how scientists have been able to prove via mutated mice that your body not only responds to taste via your tongue's sensations but also via your digestive system which rewards the body when it eats foods which have a dense caloric profile.
In other words, mice have been shown to prefer water with certain protein in it as well as sugar - even though they can't actually taste either.
One of the commenters, pjs_boston had something interesting to add as well:
It is not surprising that rats prefer sugar water to unsweetened water, even when they can't taste sweet. After all, eating sugar in that absence other food compounds results in rapid absorption of the sugar and a corresponding sugar high. The subsequent release of insulin causes a blood sugar crash, which spurs further food cravings.
This is the basic tenet of the Atkin's diet - that excessive consumption of refined carbohydrates causes "carbohydrate addiction". Like drug addiction, there is a pattern of a high followed by a crash which is relieved by further consumption.
The rats learn to prefer the sweetened water because they become addicted to it. I'd bet if the researchers had put cocaine in the water they would have seen the same result.