The move isn't surprising at all. Nokia bit off quite a bit, and could be seeking a substantial amount of cash from Apple. Nokia is primarily running on the assumption that the patents which other companies license from them apply to the iPhone, though it doesn't take much speculation to assume Nokia's lawsuit is a direct result of slipping sales. Clearly they see Apple as an up and coming threat, and want to at least capitalize on their growth.
Apple also noted this isn't their "official" response -- only that they don't intend to roll over. It remains to be seen how Apple will defend themselves. Given the current state of the smartphone market, this could be an important and interesting case to watch.