From what I've understood so far, if Nalby wins this tournament (and I'd say there is quite a chance that will indeed happen!), he'll be #9 in the race. If indeed Roddick pulls out, that means... you guessed it.
And if he doesn't win and Roddick withdraws then Murray would be in?
The only place I've really heard questions about whether Roddick will be there is from the British press. I'm sure they're hoping for any scenario that would slide Murray in.
What a joke though if Fat Dave some how ends up in Shanghai just because of two good performances at the end of the year when the other top guys are spent from performing well over the course of the entire year. But Roddick obviously only cares about DC right now so he may decide not to make the trip to Shanghai and I can't see the ATP putting any pressure on him to go. He pulled out of Madrid and Paris and it was no big deal.