Will this phone work using a Straight Talk At&t sim card, is it a mini sim or micro sim? And will I be able to get the 8.1 update if it ever comes out and my phone is on Straight Talk but not unlocked.
I see no reason for it not to work unless you manage to get the Sprint version.
I don't know what kind of SIM it uses.
You will be able to get the 8.1 update whenever its released by AT&T or Samsung directly depending on whether the phone is AT&T-branded or not. (Also, the Developer Preview is a valid option)