Wait a second ...
This is ridiculous for two reasons, the main one being that he easily could have pulled the pin, thrown the grenade and also fit into the coal chute and, you know, survived. He says he's staying behind because the zombies "won't stop," but judging from the explosion caused by that one grenade, there's no one left to keep going after them.
We're no experts, but they seem to be stopping. And burning.
Apparently, in 2012 grenade explosions are going to be huge, as the explosion seems to kill everyone in the room who's not in a coal chute. So setting off the grenade and then running would have been the same as setting off the grenade and just standing there.
Some will argue that Neville was sacrificing himself so the zombies would be appeased and the sidekicks could get away with the cure. This leads to problem No. 2, which is that Neville is a supergenius who took months to find a cure. So what are the chances that anyone else among the one percent of survivors will understand anything of what he did to create the cure (and with only one vial of cured blood to work from)?
There's a giant science facility behind the quaint little church.