Predictions range from a reception apocalypse to "most stations will not have perceptible loss." The general consensus is that secondary and tertiary market stations will have "signifigant erosion."
My general feeling is that the low number of stations will prevent the apocalypse, but that things are worse than the boys at HD are letting on. Part of the problem is that there isnt' a practical way to measure the IBOC interference of skywave reception because propagation is totally reliant on highly varied atmospheric conditions. However, pretending that it's not there is even less practical. So tune in and listen, this could be exciting.