If it hasn't already happened to you, someday it will: You'll pick up the phone or open an e-mail and suddenly get a blast from the past. Out of the blue, you'll hear from someone you used to know. It might be a former classmate, hoping to see you at the next reunion. Or a colleague from a previous job who's passing through town. Or a teenage crush who looked you up on the Internet. Has this ever happened to you?