The statement on Apple's website couldn't be more direct and to the point. "Other companies must compete with us by inventing their own technologies, not just by stealing ours," said Bruce Sewell, Apple's General Counsel and senior vice president. That's about all it says, really. Interestingly enough, Nokia said more or less the exact same thing back in October.
Apple hasn't specified yet which 13 patents Nokia infringes upon, but in all likelihood, Apple is probably right. However - so is Nokia. In this world, everybody infringes upon everybody else's patents, which is why you see few large companies really going head-to-head when it comes to patents; it's usually smaller companies out for money preying on the large ones.
We'll probably see some flexing of the muscles in the coming years, before all this will end in some sort of settlement, never to be heard of again.