A soft hybrid system not the full on "washing machine" Toyota Hybrids. Sounds alright to me. Handling could be a worry though, Hybrids are heavy, the Mustang EB is already quite heavy.
If it was still a V8 hybrid and there were, say, various drive modes from full on performance using electric enhancement (La Ferrari style) at one end of the scale to an eco mode at the other where fuel efficiency was maximised, then bring it on I say.
The additional weight in an already heavy car would certainly be an issue though. The s650, which has now been canned by Mr. Hackett, would (as I understand it) have made much more use of aluminium and hence would surely have been the ideal basis for a hybrid car? From what I've read I don't think you'll be seeing the words 'Hackett' and 'progressive' appearing in the same sentence anytime soon.
Straying off-topic a little, what's going on with the SUV/truck only thing in North America?! What happened to the 'Ford Global' initiative (or whatever it was called), where every product was meant to be sold in every market (which is why we got the Mustang in the UK in the first place, let's not forget)? Seems to me like this change of direction is a very short sighted approach.