Forstall was one of Jobs' favorites for a reason. There would have been no iOS app store without him. Jobs originally wanted a closed system, but it was Forstall that kept arguing with him about the benefits of bringing in third party developers, and finally Steve agreed.
And for all the comments about skeuomorphism, it may be helpful to rememer that most of that came from Steve; Forstall merely kept it going.
I'm not saying there isn't a potential upside to having Ive manage SW along with HW, but it's not quite clear just yet that this is an unquestionable win. Sometimes it helps to keep people focused, to not spread them too thin. Ive is a remarkably capable designer, but he's only human.