Barks are one of the way in which dogs communicate. For example, some barks mean loneliness and can be methodically distinguished from other kinds.
Our experiment showed that dogs can perceive the difference between barks originating from different situations, thus barking is perhaps a communicative tool not only for dogs to humans, but for dogs to dogs as well.
Dogs can discriminate barks from different situations Maros, Katalin et al.,
Applied Animal Behaviour Science , Volume 114 , Issue 1 , 159 - 167, http://www.appliedanimalbehaviour.com/article/S0168-1591(08)00042-7/abstract
Furthermore, the classic "aggressive warning" of a dog is a snarl or a growl, not a bite, and even this is not always associated with biting -- in other words, not all dogs that snarl, bite.
Owners of dogs that growl and snarl are often worried about having an aggressive dog. Growling isn't always serious. Dogs sometimes growl in play with each other, and may growl in play with humans. There are also conversational growls, like grumbling comments, such as 'do you really have to groom me?' which don't lead to anything. The dog isn't really spooked or angry, just grumbling. It's when dogs are spooked, very aroused, that growling starts to become a problem.
Dogs: Behavioural Problems http://www.infopet.co.uk/pages/0130.html
In conclusion: there are many reasons why dogs bark, and aggression is only one of them. Biting is more associated with other signals, like growling, and even in those cases, it depends. Therefore, to an untrained human, it is probably true that dogs that bark seldom bite, furthermore experienced trainers and dog owners may be able to tell the difference between behaviors and predict which bark could lead to a bite.