As we have seen for months now, the threat of a US downgrade is no longer trivial. Months ago Chinese officials expressed their concern and it such speculation is common fare both online and in newspapers.
Now the strongest signal yet that sovereign debt (even of the US and UK) really is no longer default risk free:
Britain Warned On Credit Rating - washingtonpost.com:
"A leading credit rating agency cut its outlook for Britain yesterday, moving the country a step closer to a downgrade and highlighting the vulnerability of the United States' own top-notch rating.
In cutting its outlook for Britain's sovereign rating from stable to negative for the first time, Standard & Poor's cited debt levels approaching the size of the nation's gross domestic product. While S&P reaffirmed Britain's actual long-term credit rating at AAA, its statement was effectively a warning about massive government spending."
BTW not that anyone asked my opinion, but I still think monetization is much more likely than default