An interesting article:
Advertisers on the Internet know who you are; they know where you are with your mobile phone; and they can track your interests and send you advertising despite efforts on your part to avoid their attention.
The good news is you can easily find out if this tracking is taking place.
The better news is that not every carrier inserts tracking codes in your data stream. The initial details of this practice were first revealed by the non-profit journalism group Pro Publica, which discovered that Twitter is making use of this information from Verizon wireless phones as a way to deliver advertising.