There is a difference between a willful patent infringement and an un-willful (not sure what the real term is) patent infringement. Both are patent infringements, and both carry penalties etc., however a willful patent infringement carrier a much higher penalty. So, even if Apple didn't know about it, it could still be infringing on Nokia's patents. This is exactly the reason why patent attorneys constantly tell folks in the R&D to never go look or read other's patents, so that if they are ever sued in the future, they can at least claim that it was not a willful infringement.