Anyone that knows the publishing world knows that revenue from subscriptions are NOT a major source of income... directly. You want subscribers not for their money but for their names to appear on your subscription rolls. Why? Because that allows them to raise their advertising rates which is where they really make their money!
They'll practically give away a subscription to raise the price of a full page ad to the next bracket.
The iPad's subscription model will make them more money - if they know how to be business people. If they don't know how to adapt to this new business model... well then they'll go the way of the buggy whip manufacturers.