Well, for us, the concern is that they might not be able to fly around it; hence, if they cancel NW11 the night before, we obviously wouldn't have a ride back to NRT.
From SIN, it would be be easy enough to fly SIN-AMS-SFO to get home as an alternate. I just spoke with reservations and they are not currently planning on any disruption in service.
Hopefully they are not being overly optimistic.
Steve
ps -- a volcano now? holy moly (although it doesn't appear to be an issue)