If the Supreme Court strikes down the healthcare law on the ground that the mandate was a "penalty" not a "tax," doesn't that mean Congress could enact exactly the same law the day after, throw in the word "tax" a couple of times without changing any substance, and it would be constitutional?
I am guessing that, if they do strike down the healthcare law, they will want to come as close as possible to the Bush v. Gore formula of trying to issue a one-off decision with zero precedential value. I doubt they'd have the nerve to state this expressly again. But given that many of the Republicans on the Court favor broad federal powers when they like what's happening (or who did it), why would they want to create any less flexibility for themselves the next time around?
Basing the reversal purely on word choice would have this desirable quality, from their perspective, by offering a roadmap that could be used to prevent the decision from serving as a significant precedent.