According to several matching reports from 2chan, Grimmjow's death is official. In his interview in the Jump Festa, when Masakazu Morita (Ichigo's VA) brought the arrancar arc into question and why it lasted for so long, Tite Kubo (Author of Bleach) revealed that Grimmjow is dead, and that he was actually supposed to die earlier in the series. It was the fans support that kept him alive for so long.
The japanese reports confirming Grimmjow's death:
583 ：名無しさんの次レスにご期待下さい：2010/12/18(土) 21:12:53 ID:VoJ+JBMP0
My own’s memory
Morita/Oririn’s question “As of this whereabouts?”
“Past halfway.” Answers sensei.
Then in that case [It needs at least 10 more years to continue.” Said Morita.
As regards to matter that arrancar arc was longer than as planned.
Then in that case, character’s unexpected activities were put in there? Morita asked.
I planned Grimmjow to die earlier, but when I drew it, I saw there was no emphasize at all, so I added activities to it.” Said sensei.