apart from the effect on Obama's health insurance regulation law, if the justices use this case to over turn "Wheat", it will make Citizens United look tiny. The commerce clause and the federal governments power to regulate commerce is the underlying basis for much of modern federal authority, from the FCC to the EPA and FDA among others. Heck much of the civil rights legislation is based upon it.
They could go narrow. Many, really on both sides of the aisle, have thought Wheat went too far. That it should have stayed limited to ACTUAL interstate commerce. There are good arguments on both sides. But even this would be huge. I do wonder if the court has the moxy to actually do this. God knows Scalia and Thomas do, I just wonder if any of the rest of them would.
The biggest reason I would think that they wouldn't do this is because the large corporations, especially the multinationals, actually LIKE the feds having all the authority. They DON'T want to have to deal with 50 states worth of regulations.