Many employers perform criminal background checks, credit checks, psycological and aptitude testing, and of course drug screenings when seeking potential employees. If you think they aren't typing your name and email address into Google to see what pops up, you're naive. Blog posts, forum posts, MySpace pages, and any number of things you type on the Internet can and likely will be used in a hiring decision. And of course with the Internet Archive's Wayback Machine, some things might end up being out there forever.
It shocks me what some of these kids post nowadays, especially on MySpace. A lot of those folks are in for a shock when they enter the professional workforce.