The NYT has a good piece about cell phones practices:
In an age of information oversharing, the mobile-phone number is one of the few pieces of personal information that people still choose to guard. Unwanted incoming calls are intrusive and time-consuming and can suck precious daytime cell-plan minutes. And the decision to give out a cell number can haunt you for years, as people now hold on to the numbers longer than their land-line numbers. (...) Some people have found a way to avoid compromising the sanctity of their cellphone without committing the modern sin of being unreachable. Instead of giving out her cell number, Ms. McClain has recently been dispersing what has become known as a “social phone number.” This is a free number that is as disposable as a Hotmail address. A handful of Web sites are creating these mask numbers, which can be obtained in nearly every area code (...) Mr. Wisk (creator of the social phone number provider PrivatePhone.com) said a person’s cell number has become the most personal, “the last one you’d give out.”“Now for so many people,” he said, “it’s the only number, and it corresponds to an object you have on you at all times. It can be a disruptive technology. Having a number that goes straight to voice mail is less intrusive.”
Why do I blog this? I tend to agree with the idea that cell phones # are like and ID and some people do not want to give them up. It's interesting that more and more service will be built around this phenomenon (counter the disclosure of information done in the past).