[Lazare]

That's an open question. The legal term is "severability". Sometimes an entire law (or contract) will be struck down if one part is found to be illegal or invalid. Other times just the invalid part will be cut out, and the rest will still be enforced. It can go either way, depending on very specific details of the law (or contract), drafting history, previous precedents, etc.

Often, a law (or contract) will contain a severability clause specifically saying what will happen. (Contracts almost always say that the contract should stand if any clause is found invalid.) The problem here is that the ACA's drafters (perhaps foolishly) did not consider the possibility of a constitutional challenge, and did not include a severability clause into the law.

If the mandate is found unconstitutional, the court will also need to rule on the severability question; it's very uncertain at this point which way they'll rule (if they have to rule at all).

(Bonus question: Do we want the ACA struck down if the mandate is struck down or not? The law does a lot of things, some of which are quite good, and would still work just fine without the mandate clause. On the other hand, guaranteed issue and community ratings without a mandate might completely destroy the health insurance market, without replacing it with anything else, which would be a disaster. The Court is highly unlikely to rule that it's partially severable, so we'll probably see the whole ACA struck down, or only the mandate. In theory, Congress could fix it either way the next day...but in practice it's likely to be terrible deadlocked. Messy.)

[bunderbunder]

I rather hope the bill stands or falls as a whole. The individual mandate is one of the lynchpins of the bill. Without it, the bans on annual limits and discrimination against people with pre-existing conditions that are coming in 2014 will probably force premiums to increase considerably. Admittedly there's plenty of good stuff in the act beside all that, but I fear it's still better to lose everything in the bill than it is to lose the individual mandate but not the things that need the individual mandate to be feasible.