SportsAgentBlog notes that there are a few possible collusion cases available to the MLBPA, but no grievances have been filed:

Why is the MLBPA not taking action against the MLB based on these potential claims? Is there not enough evidence? Has the association felt that labor peace is more important than filing a grievance on behalf of a few players who may not have strong cases? Do you see a possible grievance being filed any time in the near future?

One reason might be to keep dirt under the rug. I assume if the MLBPA files a grievance, there comes a period of discovery on both sides. In regards to the 2002-2003 case and the Bonds case, I would bet there would be a lot of questions directed at players about steroids during depositions. Maybe MLB doesn't want that to come out.

The other (and probably more relevant motive) is that the players don't have a good case. 2002-2003 was a down time, with the economy taking a big hit after 9/11. Also, like this year, there were a large number of good free agents seeking jobs, which tends to hold the price down. Even Bonds, who has the best case of anyone, is going to have a tough time proving he was blacklisted. With an indictment hanging over his head, any single GM could decide on his own Bonds wasn't worth the negative publicity.