SAN FRANCISCO, June 25 — Cellphones are chock-full of features like built-in cameras, personalized ring tones and text messaging. They also gave a real boost to Kenny Hall's effort to cheat on his girlfriend. Mr. Hall, a 20-year-old college student in Denver, decided in March to spend a weekend in nearby Boulder with another woman. He turned to his cellphone for help, sending out a text message to hundreds of other cellphone users in an "alibi and excuse club," a network of 3,400 strangers who help each other skip work, get out of dates or give a loved one the...