Massa as close to Vettel as possible is their best option, not saying they want to or will, but its an option to take him out, its not entirely possible they won't hit each other in a completely fair, entirely unintentional attempt to overtake(though it would never be read as such).
Either way, Hamilton pole to finishing first.... not if Mclaren have anything to say about it, Webber, kers/alternator, Vettel, alternator, there are plenty of options for any of them to DNF and with rain, anything can happen.
Especially if people are trying to overtake chances of cars near the front spinning is relatively high, and if it starts dry then turns to rain... anything can happen, if it pee's it down on one side of the track and you've just past the pits, and have to inch round while someone 20 seconds behind puts on wets and burns around, you can gain 10 places, or you may not make it round the wet section. With rain involved, anything can happen, from Alonso starting brilliantly and being 1st by the third corner, to Alonso being at the back and winning after it starts raining, who knows.
I hope it does rain just for the chaos that will come from it.
I wouldn't bank on Massa being behind both Webber and Vettel past the first few corners anyway, Massa and Alonso are beastly off the line, and Vettel/Webber usually are anything but brilliant off the line. If Massa can get ahead in their quick first stint and hold them up at all Alonso has a great chance. Red Bull's might go tactical off the line though, as in, hold together and block the road so Massa can't get past, if they gang up on Massa though, they could leave a gap for someone else.