In a report in the AJC, Chris Vivlamore states what we've all assumed since each of their injuries drug on past a few games: Jim Slater, Freddy Meyer, and Patrice Cormier are all out for the rest of the season. All three of them have either concussion or concussion-like symptoms, and with there only being seven games left and no shot at the playoffs, everyone figured that they'd be benched for the rest of the year.
Jim Slater has been out since New Year's Eve thanks to a concussion inflicted in that night's game against the New Jersey Devils. When I spoke with him at the Valentine's Puck photo op, he said that he was judging his recovery and progress against Sidney Crosby's, who sustained a concussion a day after Slater did. Sid's back and skating now, and should be ok for the playoffs soon. Slater's concussion was possibly more violently inflicted and probably more severe. With the new league-wide concern for player health on the heels of not only Crosby's concussion but also Marc Savard's, no team wants to risk further damage by rushing anything. Jim Slater is an integral part of the fourth line, and is also a great energy guy that you want out there. His dedication has been missed in many games that the team's "mailed it in" in. Also missed of his is his face-off ability. His 61.5% effectiveness being gone (as well as Rich Peverley's 55.5%) is part of the reason that the Thrashers are winning less than half of their face-offs on the season.
Cormier sustained a concussion (probably) on February 17th, when he found himself in an accidental head-on-head hit with Ed Jovanovski, who suffered a fractured orbital bone in that same hit. The team hasn't released anything past "upper body injury," but chances are good it's a concussion.
Freddy Meyer's injury is a bit more nebulous. He collapsed at center ice in the second period of the Thrashers' 4-2 loss to the Calgary Flames on February 3rd. He hit two players in rapid succession prior to his collapse. He was screened by doctors and a concussion was ruled out, but he's still been sitting since that game.