Once upon a time, the NFL would not let players not under contract serve suspensions unless and until they belonged to a team. And so men like receiver Andre Rison, whose NFL career ended with a four-game suspension hanging over his head, had significant trouble getting a new job, since the first order of business would have been to exile him for four games.
The rules have since changed. As NFL spokesman Greg Aiello confirmed to us via e-mail, players not under contract can nevertheless serve their suspensions. So, for Chargers receiver Vincent Jackson, he won't be required to sign a contract before serving his three-game suspension. Instead, he'll serve his time from Week 1 through Week 3, regardless of whether he signs his tender offer.
The same rule doesn't apply to players who are under contract and holding out. Aiello explains that, in those situations, the suspension is tolled under the player under contract reports.