There's no 3G/3GS on Verizon and exactly zero iPhone 4 Verizon customers are eligible for a subsidized upgrade yet. So it only affects the tiny number of customers who are going to pay $600 for an out-of-contract upgrade. The vast majority of Verizon iPhone owners would have to wait for their subsidized upgrade date which for most is March/April 2012 at the earliest.
For AT&T customers things are a bit more tricky. Any iPhone 4 owners are in the same boat as Verizon customers. Most won't have a subsidized upgrade available until late December. (if you have a more expensive plan with AT&T your upgrade date is earlier) This also goes for people who bought the 3GS in the last 14 months. The people who opted for the 3G in the last ~20 months decided to buy a low end model. For them the iPhone 4 or 4S would be the natural upgrade path and a massive upgrade.
No doubt there are some risks but I doubt Sprint would have a very long exclusivity period so even if Apple takes a beating for a month or two it will be of little consequence compared to $20B in the bank.