First off, let's straighten out the "good for all debts public and private" asopect of cold hard cash - one needs to focus on the "debt" part of that statement. You are not walking into an Apple store with a "debt" to pay...rather to enter into a contract to purchase an item they are selling...and if you don't like the Terms of Sale (debit/credit only) - don't enter into the agreement and do not buy the iPad.
OK, so that does away with the socialist(?), illegal nonsense. It's not illegal to set Terms of Sale. Now, had you "owed" ATT money on your 3G account, they, on the other hand, would be compelled to accept cash as that is paying a debt.
BUT, and as Jeremy Clarkson would say, "it's a great, big, full-bodied BUTT", it is beyond inane to not accept cash. Foolish. Arrogant. Elitist. STUPID.
Not a bright move, Apple.