Before people had D7100's in their hands, KR mistakenly posted on his site about some of his readers having D7100/Sigma lens compatibility problems.
I'm convinced he was referring to the D7000 (which DID have some Live View issues with SOME Sigma lenses, that required new firmware). I own 2 of the affected lenses (made BEFORE the D7000 was released, and had the firmware updated for the D7000...
I've now tested my Sigma 150/2.8; 8-16mm; and Tamron 17-50/2.8 (non OS/VR edition). ALL work perfectly on the D7100, using the viewfinder focus, and LiveView modes.
In Live View, AF-S, single point mode, it sounds like the camera sets and re-sets focus, but the focus is dead-on.
In regular mode the focus is fast and dead-on (seems faster than LiveView).
My Sigma 50/1.4 also appears to be fine, compatibility wise, but it's not in my hands right now to double-check the LiveView aspect.
Now, if someone has Sigma lenses made BEFORE the D7000 was released, and didn't get the D7000 firmware, I don't know what their results will be on the D7100.
Sigma lenses made AFTER the D7000 was out (with that firmware), may well be totally fine on the D7100.
That's really good news, thanks. I've been posting about this issue elsewhere on here and can now send my 50-500mm OS and 105mm OS to Sigma for re-chipping, so I can use them on the V1 where they don't work. Sigma now have a fix and guess they will work on the D7100 after reading your comments.