ARG has Hillary Clinton up by 26 points in South Dakota. The only other poll of the state had Obama ahead by 12. But that poll was conducted two months ago and had an extremely small sample size, so ARG is essentially flying blind here.
This much for certain: Clinton's South Dakota stock is a ridiculous bargain on Intrade right now. But South Dakota isn't all that idiosyncratic a state, and 26-point win just doesn't make any sense in the context of what we know about the demographics of this race. The national tracking polls are fairly stable, and the campaigns aren't behaving like South Dakota is a 20-30 point Clinton win.
As far as methodological nitpicks go, I don't really have any, because ARG does not disclose all that much about their methodology. It's considered less than ideal to poll entirely over a weekend, but that's something that happens fairly routinely (by ARG and most everyone else).
It's either a genius call or ... something the opposite of genius. You know on which side my bread is buttered.