So they are going to disable the block button when you have a shield and magic equipped?
Doubtful, as that wouldn't make any sense. As I understand it, if you have a shield equipped, tapping the button for that hand allows you to block, while holding it down allows you to do a shield bash, no logical reason why having a spell in the other hand should prevent you from doing that.
Now, it's been said that you can't block with a single handed weapon and spell, I'm not sure if that's true or not, but I could see it being so due to there not being enough buttons for it, but we'll see.