In a post on Nationals Insider, Mark Zuckerman walked through the ramifications of various debut dates for Bryce Harper, assuming that occurs sometime in 2012. Because that question only comes up all the time here, I thought I would cross-post to it as a public service (while giving Mark credit where credit is due). From Mr. Zuckerman:
Just to clear this up, here are the different scenarios for when Harper debuts and how that affects his arbitration/free agency status...
-- If Harper is on the Opening Day 2012 roster (and if he stays in the majors for six years without any demotions), he would become a free agent after the 2017 season.
-- If Harper starts 2012 in the minors and is promoted before roughly mid-June, he wouldn't become a free agent until after the 2018 season but would qualify for Super-2 status in 2015. Though he's already scheduled to earn a $1 million salary that season, there's a provision in the contract that allows him to earn an arbitration-level salary if he qualifies. So this scenario could cost the Nats several million dollars.
-- If Harper starts 2012 in the minors and is promoted after roughly mid-June, he wouldn't become a free agent until after the 2018 season and wouldn't qualify for arbitration until after the 2015 season. This scenario both saves the Nats money because it eliminates the Super-2 possibility and ensures another season of team control before Harper can become a free agent.
I know it's all confusing -- believe me, sometimes I can't even get it all correct in my head -- but hopefully this helps clear it up a bit.