Well, I've nearly plowed through about 2 gallons of this stuff!
So few days ago I started another small batch of grape, and a small batch using an Apple/Kiwi/Strawberry juice concentrate. What I've found is that if I close the lids tight on these once they're in the fridge they retain a nice amount of carbonation. I even bottled 2 bottles of it in corona bottles just to see what would happen. I left them in the fridge and they haven't exploded yet, so I'll probably drink them this weekend.
Oh, one other thing I've noticed. Now that I am down to the last glass or two of the grape batch I made, I think it is drying out a bit, and getting a little more sour. I think that what's happening is that the fermentation is slowed way down, but not stopped completely.