They are building it. I got to take a short test drive in the 425 hp version. It had pretty decent power and certainly handled better than my old muscle cars with similar power. I also got to drive a prototype Camaro a few months ago. I do believe (but am not sure) GM will build it. They have lately been talking making it an e85 car, which I think would be a big mistake.
I've been seriously thinking about buying me a "driver" muscle car. I have driven several different types of the Mustang but, for some reason I can't just pin down, they simply don't do it for me. Unless you go with the Saleen or Raush (huge increase in price), their performance really isn't impressive. My BIL has one of the new Mustangs and was quite depressed when he could only pull about a car length on me in my little Acura. Worse yet for him, on the interstate, my Acura ran off and left him at high speeds.