Contact Sprint and ask them if you cancel service now (less than a month out from your end date) would you incur an ETF, and hopefully you will get the same answer I did. I am also switching from Sprint to T-Mobile with a contract end date of 10/06/2013. When I contacted Sprint regarding what sort of ETF I would have to pay if I cancelled my contract before that date, I was told that if I waited until after 09/06/2013 there would be no ETF. I received this info via chat so I have the transcript for proof, and I do plan on verifying this one more time (via chat, so I have the transcript for proof) before activating my T-Mobile service.
I did let the agent know how unhappy I am with Sprint's service, not sure if that helped or not.
Good luck!
My sister did some more digging on the Sprint site and it looks like I can leave Sprint as soon as I receive the iPhone from T-mobile. Something about less than a month to go on the contract..