Nope,
the original iPhone was not subsidized, we paid $599 (8g model) for a 2 year contract, but this time around the 8g model was going for $199, because AT&T agreed to subsidized the iPhone and pay off the remaining $400 to Apple if you agreed to sign up once again for another 2 year contract. for that reason, if another iPhone were to be released next summer you will still be under your original 2 year contract until 2010, so I doubt that next time around we will have the same luck to to be allowed to upgrade again.
thats my guess, but no one here on the forum knows, things may always change, and no one even knows if another iPhone will be released again next june/July. apple just releases things