More and more people are purchasing cell phones, and in doing so, many people are eschewing traditional land lines. This can cause confusion in regards to looking someone up, because you can't just turn to the White Pages as you did in the days of land line dominance.
National Cell Phone Registry
A new cell phone search service allows you to look up people's cell phone numbers nationwide. It also allows you to look people up by city/state, while allowing "reverse lookup features," where you type in a cell number and learn who owns that particular phone number. The site is available at cellphoneregistry.org, and it also provides information on finding land line numbers as well.
A similar service in Great Britain — 118 800 — allows you to search for people. While it doesn't provide you with a mobile phone number, the service reaches out to the person in particular with your information, thereby allowing them to reach you at their convenience. The site is relatively new, however, and doesn't guarantee 100% accuracy.