Ed Kilgore believes many of the conservative Southern states won't:

The sad truth is that Republican governors and state legislators have been claiming ever since ACA was enacted that the expansion, even with the “sweet” super-matches, would bankrupt their budgets. And the even sadder truth is that many of these solons don’t think of this as primarily a fiscal issue, but as an ideological test of their hatred of the “welfare state.”

Yglesias differs:

The way I see it playing out, in most places local health care providers—hospitals, doctors—plus the national pharmaceutical industry will exist as a powerful lobby in favor of expansion. More Medicaid equals more customers. Certain classes of low-wage employers should also favor expansion. That means the greatest opposition to expansion over the long-term may come not from places like Texas, but from the low-population states of the Plains. Why? Because those are the places most likely to be suffering from shortages of doctors and other health providers. In a place where people worry about provider shortages, Medicaid expansion won't do existing providers much good. And the existing set of insured people will worry that completely aside from financial issues, Medicaid expansion will mean fewer doctors' appointments and hospital beds to go around.

Florida and South Carolina are already promising to opt out. Josh Barro weighed in earlier.