The SM-G900T is definitely SIM locked. I ended up just buying the unlock code from CellUnlocker.net and amazingly they were able to get the code on launch day. Yeah, it costs money, but they came through with the code.
Also, lucky for me, since I guess I was one of the first to buy a code for the G900T, I got an email from them saying that they were looking for footage of people unlocking their GS5s, and if I could provide a video showing the full unlock process, entering the code, and activation on another network, they'd refund the purchase price. I don't know if they're still offering that, but I took advantage of it and I did indeed get my full purchase price refunded, so in effect I got my code for free.
Every OTA update for every Nexus and Google Experience device since the Motorola Droid at http://www.randomphantasmagoria.com/firmware
LG G3 (LG-LS990 / Sprint)
:: LS990ZV4 (Android 4.4.2 / KVT49L)
Samsung Galaxy Tab 4 8.0 (SM-T337A / AT&T)
:: T337AUCU1ANF4 (Android 4.4.2 / KOT49H)
Samsung Galaxy Note Pro 12.2 (SM-P907A / AT&T)
:: P907AUCU1AND7 (Android 4.4.2 / KOT49H)