The rumors I heard were in the neighborhood of about 2-3 weeks ago???
It makes sense, I actually heard about him leaving from one of my Ford contacts before my GM contacts. I don't know all the details of how much Ford owns of Mazda, but she didn't know where F.V. was going either, but had "heard that the designer of the Solstice went on to greener pastures". I did think it odd that I heard of it outside GM first, but I'm starting to understand why.
If he left a few weeks ago, I'm sure he was negotiating with Mazda long before that, depending on the position he is getting at Mazda. I know policy at the big three is pretty much an escort out the door just about right then and there if you quit and go to work for a competitor - no such thing as a 2-week notice.
I'd bet it's especially so in something as sensitive as GM's Design Studio: Think about it - that dude has seen everything on GM's plate, everything they've been thinking for strategy for the next 5-10 years!!! THEN he leaves GM to go to a (semi)Japanese competitor? WOW.
I bet it's a lot like if you find out your significant other has had and affair and is leaving you - you wonder about the conversations you had, the times you spent together, and then start thinking "was the other person involved when I said this or that?" or "I wonder if they were talking to the other person on the other end of the phone when I was <making dinner, in that design meeting, fill in your activity here>."
Even with the negotiation time, unless he was leaking confidential information to Mazda ahead of his negotiation, I doubt he had time to influence the current Miata.