This announcement comes a little surprising. Aside from my personal opinion about taste and functionality, Mr. Forstall nonetheless guided iOS in a direction. And I completely understood why he was chosen. On the other hand, I never understood why he was on stage so often. In my personal opinion, Mr. Forstall tried to act like Mr. Jobs on stage. To me, this felt unauthenthic and untrustworthy which probably is why I personally never built excitement for anything he presented.
Personal taste aside, I doubt that something like not signing an apology-letter has something to do with it. Maybe it was just the last drop.
If on the other hand what is rumored about apple being divided into camps is true, his leaving (or being pushed) is totally comprehensible. Everyone can be kicked (pushed, mobbed) out, even the big ones. That is sad and I'm sure, all these posts here only scratch the surface of what is the true reason behind it all.
This announcement sounds to me that there were very personal (not so pretty) things going on which lead to this decision. I wish him strength for the future.