the 100$ price reduction is just for the reference board I think. shouldn't affect custom boards so much. And 599 for a custom is already very cheap.
Seems to be for the Custom-Designs as well... a german store (caseking.de) reduced the prices for the 1080 from EVGA today... here an example: https://www.caseking.de/evga-geforce-gtx-1080-gaming-acx-3.0-8192-mb-gddr5x-gcev-259.html
Well I have the ti bug, so a 4 month old EVGA GTX1080 FTW with EK full water block will be on sale soon.