It is confirmed that the T-Mobile S5 is bootloader unlocked and can work on ATT network and get ALL frequency bands that ATT uses. Just check out the T-Mobile S5 Subforums.
They already have root, recovery via TWRP or some others, just not a lot of Roms yet. I follow the T-Mobile and ATT forums in case I upgrade from my S4.
Sent From My Spiderman,Ironman,Red,Dark Blue,Green, GreyedOut BadAss Themed I337
I can also confirm that T-Mobile's bootloader is unlocked. I have a T-Mobile S5, which I have rooted and installed TWRP and am running AllianceRom (from the international forum, so you are not limited to the roms in the T-Mobile S5 forum).
I actually also have sim unlocked the phone and am using Straight Talk (AT&T) service. I get full AT&T speeds, including LTE.
Can I just go to a T-Mobile store or online and purchase an S5 outright? Given that it has an unlocked bootloader? I heard that some one people got the Verizon or Sprint version and they were able to use on the ATT network (apparently some of them are unlocked) but apparently having gotten the Verizon version means they can't use T-Mobile ROMs. How much different is the hardware? I feel silly for asking this question because I'm pretty confident I know the answer but it's a lot of money to shell out: doesn't unlocking the bootloader mean you can literally install almost any type of OS or ROM or Kernel to your device? Assuming you can support it yourself or the community has created ROMs that support it?
Last question of the least importance: Anyone running a hybrid HTC Sense from the M8 on their S5? I know there aren't many T-Mobile ROMs for the S5 yet, but I figure this is the best place to ask them since you guys really know what you're talking about.
Thanks everyone! If I DO come across the answers to my own questions, I'll post them myself.
BTW! if anyone wants to buy an AT&T LG-G2, I have one in perfect condition that is sparingly used. I'm not going to post on swappa.com until I know for sure what device I'll be buying. However, I am willing to make a listing for anyone that truly wants an LG-G2 so we can make the transaction and I can ship the LG-G2 immediately after everything goes through.