It was inevitable, I suppose, but Bob Sanders is done for the year, and probably forever with the Colts.
The move came as the Colts finally just flat ran out of room to stash him. Given the admission recently by Bill Polian that the team could possibly have gotten Anthony Gonzalez back from his knee injury, the move to keep Sanders off IR so long is an especially damaging one.
At the time, the Colts most likely felt their best chance to win this season would come from improved safety play in the playoffs. The recent losses have left the team with no room for error however, and they can't afford to go into tomorrow night's game even more short handed than usual. At the time the decision was made to put Gonzo on IR, Collie had not yet had his concussion. Clearly, if the team had known what was coming, they would have made a different decision.
Given the fact that Sanders was not yet even cleared to practice however, it seems unlikely that he would have been able to return soon enough to help. Given his outstanding contract situation, there can be little doubt that he will be released following the close of the season. While the team could bringing him back for less money, he has probably played his last game for the Colts.