[TCC]

Taken from here http://www.straightdope.com/mailbag/mdognose.html

"For starters, dogs' noses aren't always wet. Sometimes they're dry, sometimes they're runny, sometimes they're scaly. Most of the time, though, a bright, happy dog will continually lick his nose (and you, the air, and his private parts; it's not like they're discriminating) and so the nose is wet. When the dog isn't feeling well, he tends not to lick his nose, and the nose is dry. This is not a direct correlation, however, and does not mean that dry noses equal fevers. The brachycephalic breeds (bulldogs, Bostons, pugs, also known as the snorty ones) have noses set so high on their muzzles that they can't reach them with their tongues and thus the noses tend to become cracked and dry on top. The nose, known to its friends as the nasal planum, is unique in the dog in that it lacks apocrine sweat glands, as do the footpads. The footpads do have eccrine sweat glands, which is why nervous dogs and cats leave sweaty little footprints everywhere.

Some other animals do lick their noses. Their own, I mean. Cats do, but their tongues are so dry you don't get the same sloppiness as dogs. Cows are the pre-eminent nose-lickers, as they can clean a little ways up each nostril with a swipe of the tongue: truly an awe-inspiring sight and one that should not be missed on your next trip to the farm."