The lack of NA HSPA is a hardware limitation, not software. So no, there are no software updates that will allow the device to function on NA HSPA (3G)
Would an unlocked 9520 do the trick? I've had a few people ask me. I suspect that it will, BUT since it's still unreleased I don't know for sure. If it does it'll make lots of ATT & Tmo people happy.