SAN FRANCISCO -- Cell phones are chock-full of new 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. 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 cell phone for help, sending out a text message to hundreds of other cell-phone 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...