When Hines Ward held out in Pittsburgh, I remember the worst case scenario for him returning being either week 8 or 10 - I'm leaning toward 8. After that, the players lose credit for the year in regards to their pension. Unfortunately I can't find a link to an article, I may have just heard it over the radio.
Edit: Here is something, but it talks about if he sits the entire year it won't count toward his contract.
Mr. Parker is in the middle of this as well.
http://www.pittsburghlive.com/x/pittsburghtrib/s_359413.html
Parker would like to avoid such a scenario, although he offered no assurances when asked if Ward would make it to camp - or if he'd even play for the Steelers this season.
"Do I expect it?" he said. "I can use the term, 'Hope.' I would hope it would work out. I tend to be a positive person.
...
"Parker said he never has had a client hold out an entire season in his 20 years as a players representative. If Ward turns out to be the first, it could be costly, considering he could be fined for missed games, potentially forfeit 25 percent of his signing bonus ($500,000) and remain the property of the Steelers for '06, thereby missing out on free-agency. "