I think that'd end up in a humble bundle before anything else.
Is it on Mac and Linux? I know HIB requires games to be cross-platform compatible.
I'm not sure, but they could always develop Mac/Linux versions in the future, like past games have done that were included in Humble Bundles. I think a couple of the games in Humble Bundle 6 didn't even have finished versions for their Mac or Linux builds (can't remember which) when the bundle was first released.
The reason I think Dear Esther is more likely to be in a Humble Bundle before any other bundles is because it's been in the Humble Store for a while. Same reason I think Legend of Grimrock will be in a Humble Bundle first before any other bundles.
In either case, I highly doubt it'll appear on IGS first, seems too high profile for a IGS deal.