I don't think they offer unlocked phone's until a few month's after sale's begin, BUT apple state's that all phone's are unlocked? Seem's to be lot's of confusion if the phone's are locked or unlocked, and when i started reading post i became confused.
I pre-ordered my phone, and like you i didn't want to choose one of the four carrier's, but i went ahead and chose AT&T so i could get it in store on release date. I couldn't get apple upgrade program because i didn't have an account with one of the four carrier's, so i just payed the full price. I thought about getting service from AT&T just in case, but blew it off thinking it wouldn't matter, and that's the reason for being disqualified for the upgrade program. The phone came with an AT&T sim installed, so i just went across the street to AT&T and purchased a GoPhone plan. I should have tried my ST sim first. I don't use a lot of data, and was on straight talk, which is owned by AT&T. I figure if i start using lot's of data i'll just go back to ST.
Then i read a post by someone that has an AT&T, and he requested the phone be unlocked. It was approved. Now i've only had the phone since the release date 9/25, so a couple a day's ago i requested it be unlocked. Whether it was unlocked to begin with i have no idea, but i received two e-mails from AT&T at the same time. One said sorry, and the other said congratulation's follow these step's. Went into iTune's and did a backup, then Restore phone. It started with the 9.0.1 update, restored, asked if i wanted to use backup (i did) and when it was finished the window came up congratulation your phone is unlocked.
Something else i found while playing with the setting's, was that when i went to ST from AT&T i had to change setting's on my iPhone 5. Some one on YouTube said to get a sim from T-Mobile, because they will give it to you free because it's not activated, and your setting's will open to reset for ST. It worked. So i put the same sim from T-M into my new 6s+. In settings there was a new one for Carrier's listed right under cellular. I tapped it and the next window said Automatic with the radio button on green? I turned it off and it searched, and up popped up a list of the 4 carriers you see when ordering. I'm assuming that if you use one of those carrier's the phone will automatically find the setting's for that specific sim card, because when i replaced the AT&T sim a box appeared that setting's were being updated. That made me wonder if it will reset any carrier setting's, but Apple only displayed the four major one's?