It has already been stated that Nightfall will be available in the GW Store upon its release. However, I think that ArenaNet should open the purchase rights a few days earlier (but not allow access to Nightfall content until it officially releases, of course).
The reason? The GWStore will be annihiliated upron release. I bet half of the GW Population will observe the conveince of ordering from the store. That causes latency. That causes crashes. That's not plesant.
What do you think?
Pre-Release Nightfall Retail in GWStore
Zinger314