Will a Sprint iPhone 6+ work on AT&T?

Nope. Sprint doesn't unlock their iPhones for US as of the moment.

 

As indicated no. Sprint does not unlock for domestic use - ever. And yes, I am aware of the new unlock law and so is Sprint. Sprint blames it on the manufacturers so that they have a technical out to deny you a domestic unlock even though the new unlock law applies to them.

That said, even if they did allow unlocking you still have the whole CDMA/GSM thing going on. I know the iPhone 6/6+ is a world phone though so that may not be an issue any more.

In any case - you can't.

 

The new unlock law just makes 3rd party unlocking legal again. There's a voluntary unlock agreement between FCC and the carrier consortium but that takes effect February 2015.

I agree. The fact that Apple made a separate model for the US for the 6/6 Plus likely suggests it was done at the request of Sprint so they can claim it's "technically impossible" for them to do a domestic unlock. Which makes absolutely no sense whatsoever considering the same exact model used by Sprint is being sold unlocked overseas and can be used by international travelers on AT&T, T-Mobile and Verizon.

Sprint is obligated to unlock phones introduced after February 2015 for domestic use so they won't have an excuse with the 6s or whatever the next model may be called.