it would still be great if someone from GPUGRID could give us the information at which exact date XP support will be stopped

"The GeForce GTX 1080 and GTX 1070 support Windows 7 and above. There are no plans to add driver support Windows XP."

Manuel Guzman, Nvidia Customer Care, November 2016.

FYI: at the release of GTX 1080, I've successfully "hacked" the then-fresh NV driver (with CUDA 8.0) for Windows XP (x64), and used my GTX 1080 under Windows XP (x64) for crunching Einstein@home tasks. It doesn't worked with GPUGrid though. So there's no "physical" obstacle of using the GTX 10 series under Windows XP it just made up by NVidia.

I've reanimated my Windows XP x64 on my host with a GTX 1080 (with the latest Windows XP driver v368.81). (Don't be surprised if its page lists this host have Windows 10 - It has dual boot, but both OS starts the same BOINC manager, so the displayed OS depends on what I've booted into.) It's happily crunching CUDA4.2 CUDA5.0 and OpenCL tasks for SETI@Home. When it will have a lot of results, with a little luck I will be able check its performance against other GTX 1080's under different OS.

Windows XP users will notice that the present CUDA6.5 app (v8.49) will not work any longer at all after x April 2018.
The question is the exact date (x) when it will happen, as it was not declared in the original post and ever since.

The largest possible value for x is 17, because that's the date that app was deployed for the public to use.

In fact, it will be x-t, where t is the time used for internal testing between licensing and building the new app, and making it available to the public. From what I remember, that was pretty quick.

Well, it's 18th April, and the Windows XP app still working.
I ponder over how much longer it will work? Hours? Days? Weeks? Months? Years?
The longer is the better from my point of view...