There are a few reasons for this behaviour. It may develop from an initial lick at a food residue and the dog just likes the sensation and it becomes a habit.
Not so bad, you think, but it may be more sisnister than first appears.
If you wear outside shoes in the house then you carry many pollutants onto your carpets, some of which are harmful and some of them addictive. This is bad news for your dog AND bad news for you as the pollutants are now in your atmosphere too. Take lead for example - greatly reduced in the environment as we produce cleaner engines and fuels, but still there in harmful amounts. By removing your outside shoes before walking on the carpets you reduce your exposure to lead by something like 35%. Lead is a culminative poison, it builds up in your system and your body does not remove it on it's own. Milk is the only thing that can get lead out of your system - milk is not a good idea for adult mammals as their digestive system does not retain the enzymes and bacteria needed to break down the lactose in it (most humans cope fairly well with it because we are continually exposed to it).
The other major problem with lead is that it is addictive - old fashioned lead paint was the cause of much nervous sytem and brain damage in yester-years when babies toys and cribs were painted with it. It has a sweet taste and babies became hooked on chewing on it. If you have lead deposits on your carpet this could be the reason for the dog developing the licking habit. A friend of mine once used old wood to build beds for her new kennel block - six months later she had lost two of pups and a brood bitch to lead poisoning. The wood had been painted with a lead based paint and her dogs had become hooked on chewing and licking the paint off. Matters were made worse with her efforts at controlling the situation - she moved the puppy that shared with the first puppy that died to a new kennel in order to reduce infection risk (not knowing what had caused the death at the time). This was a whole new source of lead for the moved puppy who died shortly afterwards.
Many modern cleaners and fresheners that we use can have a similiar affect on behaviour so be aware of products you are using on the carpet - any alcohol based cleaner, for instance, could have a similar effect.
The behaviour may also be a sign of stress (boredom, for instance). Licking the carpet gives the dog a sensation and relieves the stress it feels - like biting your nails or sucking your thunb. Oral comfort like this is a hangover from child/puppyhood and is often accompanied by a dreamy demeanour.

In most cases it is a comfort thing and you must be careful how you react to it as the final reason for this behaviour is that it gets your attention!