I may be wrong on this, but I think there are legal reasons that Apple must give iPhone owners free updates. But since it is so new, iPad owners will probably get 4.0 for free. It is plain unjust to completely rework a device only a month old, and make users pay for the update.
That's because Apple gets revenue from AT&T each month. As such, they do not declare all the revenue coming from a single iPhone at the same quarter: it is distributed over the period of the contract. This allows them to offer the updates for free, as payment is not done entirely.