Forstall, who is being painted out of all of the success at Apple in a revisionist version if Apple's history, has been blamed for all things bad about Apple since his departure.
What is being missed is that Forstall has a reputation as a brilliant engineer who brought Steve Jobs visions into existence. Jon Ive may have put together the hardware design but Forstall was the person who made it all work.
What Microsoft needs right now is that brilliant engineering to ensure it starts delivering products at Revision 1. This hasn't been true at Microsoft, ever. Is probably the company's biggest failing.
Hiring Forstall may be a risk, but for Microsoft it's definitely one worth taking.
For Forstall, if he can overcome the feeling of working for the enemy it would be a great opportunity to rewrite history in a way that doesn't paint him quite so badly as some at Apple would like.