Pretty easy to do, actually. Just sign into a Google account, go to http://voice.google.com
, and set up. It'll give you a number, ask you where to call (your real number), and you're all set. You can even set fun filters based on incoming caller id, time, etc, etc.
Now, the only mildly tricky part: To call out, you MUST use the Google Voice app. Have it call out. It will dial you and then the number you're calling and conference them. If you do not do this, your "real" number will show on the caller id. This is just unavoidable, beyond Google's control.
Please let me know if there's anything else I can cover for you.