Thanks for the helpful information.
I've cleaned contacts on lens and body and checked again just to make sure I'm not crazy - the body drives the Tamron 19-35 and the 18-55 DA (!) just fine, but will not autofocus the Sigma. Likewise, the Sigma still focuses just fine on my K10D.
I guess I'm gonna punt and unload the 24-70. It's a real useful range for a walk-around, but only makes sense to me on FF. If it's gonna be MF only, I'd just as soon walk around with a 28/2.8. This is the 24-70/3.5-5.6 by the way, not the constant 2.8, so no great shakes.
Still wish I understood the problem, though.
Drew