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This is a fairly common myth that isn't at all true. I've been a Veterinary Tech, currently volunteer at a local no-kill shelter and am usually the first call my friends make when they have pet questions ... but I'll back this answer with cited reference.

Written in USA Today http://www.usatoday.com/tech/columnist/aprilholloday/2004-02-12-wonderquest_x.htm
Dogs' noses are not always cold, even when they're well. A dog's nose changes through the day: wet and cool one moment and warmer and drier the next. The nose wetness (or lack thereof) also changes with the humidity. The nose norm varies from dog to dog. A dog's nose, though, tends to be cool and moist just as the inside of a human's nose. "A dog has special glands inside his nose that secrete a watery fluid to keep the inside of the nose moist and to help cool the dog through evaporation," says Holly R. Frisby, veterinarian at Drs. Foster & Smith, Inc. This moisture can extend to the outside of the nose, making it shiny, wet, and cold. Also, they continually lick their noses.

And, from the American Kennel Club http://www.akc.org/public_education/healthy_dog.cfm
Nose - A dog's nose is usually cool and moist. It can be black, pink, or self-colored (the same color as the coat), depending on the breed. Nasal discharge should be clear, never yellowish, thick, bubbly, or foul smelling. A cool, wet nose does not necessarily mean the dog is healthy, and a dry, warm nose doesn't necessarily mean he's sick. Taking his temperature is the best indication of illness.