I talked to one dealer and visited another. Both had "demo" 2014. In both cases they told me that $7500 credit would not apply to the purchase. One dealer gave me a lease offer, which included the $7500 credit. So apparently, the credit is good for lease, but not for the purchase, although I got an impression that the dealer didn't understand the details and simply used the numbers from the BMW Credit site.
I think that lease is a safer way to go. Used Nissan LEAFs are cheap and i3s might not fair any better. After 2 years it will be impossible to sell it for the residual value. I'm actually considering waiting until the next summer when off lease i3s will be dumped on the market en masse and buy one of them.