User blog comment:Lemursrule/BLEACH 310 Review: S.O.U.L (sorry on unacceptable lateness)/@comment-3430212-20110227213340/@comment-893506-20110227220145

Ichimaru died from the injuries Aizen gave him after his attempt to kill him. Ichimaru himself says he was going to die from from Aizen's attack on him ("I can die and leave the rest to [Ichigo]"), and since time has passed between the end of Ichigo's fight and the recovery seen in Episode 310, Ichimaru has indeed died. What they did with his body is unknown, but what further backs this up is that despite the fact that Ichimaru was branded a traitor alongside with Aizen, only Aizen was being sentenced by the Central 46.