The best legal analysis I have seen so far is this article:
http://www.phonenews.com/content/view/2386/9/
As a lawyer, I feel the need to convey the the importance of case law. (I am not talking down to non-lawyers, I only learned this when I went to law school and simply feel that everyone should have some basic legal knowledge.)
Even if a law does not expressly say something, the courts may take a broader view of the law. If a contract provision is contrary to the law, the contract provision is void. The Magnuson-Moss Warranty Act clearly presents a problem with Apple's current stance.
If Apple intentionally detected the modifications and went out of their way to cripple people's phones, they are dead.
If the firmware upgrade simply resulted in an iBrick, Apple must establish that the modification or enhancement is responsible damaging the product in question, if they are going to deny iPhone owners any of Apple's standard attempts at service. This aspect is the likely loser for Apple because there are apparently unmodified iPhones turning into iBricks. Apple seems to have a method to restore these phones to their original state.
In summary, unless Apple can establish that the modification is the cause of the damage, Apple must at least attempt its basic restore.