The thing is, the coin option sounds silly, but it clearly obeys the letter of the law. As far as I can tell, none of the other options — other than outright surrender — has the same virtue. Failing to pay debt service would be a breach of contract. Paying contractors, and maybe Social Security recipients, in scrip would violate the law, which says that they should be paid — not given IOUs. Deciding that the president has the right to ignore the debt limit after all would avoid these legal breaches at the expense of another breach.Indeed, what else would be legal and would work? That's why Obama can be counted on NOT to do it... :-(
(Krugman provides some details here.)
UPDATE: Treasury says they won't do it. WTF happens now? And who gave an executive department the authority to override a presidential order? Stranger and stranger...