Here's my take on it:
When I first started playing the piano, I started on a keyboard. I didn't switch to piano until about two or three years later, and I think I'm fine now. So it is definitely possible to learn the basics of piano on the keyboard, because they both have keys and you play them by pressing them down and the hand positions for both are similar.
However, you can only go so far, because they are really different instruments and the more advanced you get, the deeper you get into stuff that's really specific to that one instrument. Even touch-sensitive keyboards cannot respond to your touch the same way piano keys will. With the piano, when you get more advanced, the touch really matters, as well as a lot of other details like how tense your finger is when you press it down, how quickly you do this, how relaxed... all that stuff. And you won't be able to learn any of that on a keyboard.
In other words, if you want to learn how to have an intimate relationship with the piano, if you want to learn the subtleties of the instrument instead of just how to play the notes and do dynamics and stuff, you've got to have a real piano.
On the other hand, the keyboard is an instrument in its own right, and probably has subtleties and personality of its own that I have never discovered.