Master Chief dies at end of Halo 3?

Could it be that we are going to see the end of the Master Chief in Halo 3? Judging by the last 2 commercials that were released it certainly seems a possibility.

In "Museum" Major Pawel Czernek refers to "when Master Chief armed his grenade.." which doesn't make much sense at the time until you watch "Believe in a Hero" where we see the apparent lifeless body of the Master Chief being held in the air by a Covenant standing on a hill surrounded by hords of other Covenant. We then see a close-up shot of a grenade in his hand and later he lifts his head to show he is still alive.

Is the grenade in his hand the one referred to in the previous commercial? Since he is completely surrounded by numerous Covenant and seems to be in bad shape what is one grenade going to do..? Surely not enough to get him out of that situation ....

....or will he sacrifice himself?