Bernanke has been using a new trick to keep some money out of circulation, for awhile. He is paying interest on bank's excess reserves. This keeps money out of circulation just like the Treasury selling bonds and paying interest on bonds. I think it is best to view the central bank as part of the government. In hyperinflation the debt that is due in the first year or two will be monetized. There is something like $8 trillion in Treasuries due in the next 12 months, out of $16 trillion total. If people stop buying bonds the government will have to print a lot of money. This $1.6 trillion of excess reserves at the Fed will also contribute to the hyperinflation as it leaves the Fed, just like money leaving the Treasury. The deficit of $1.4 trillion needs to be monetized as well. If people stopped rolling over bonds and banks took out their excess reserves, there could be $11 trillion in newly printed money over the next 12 months. This is more than enough to start hyperinflation.
Here is another way to think about it. Imagine that Bernanke was paying interest on $10 trillion and the Treasury only had $8 trillion in bonds longer than were 1 year or longer. There would still be about the same amount of money "off the street" so the inflationary situation would be the same. The risk of money flooding onto the street would be about the same. The interest that investors earned could be about the same.
Clearly you could also imagine that the Treasury sold $1.6 billion more in short term bonds and the Fed was not paying interest on any excess reserves. Again this would be the same in terms of interest earned and money off the street. It would also be the same in terms of danger of money flooding onto the street. So it is really the total owed by the Fed and the Treasury that matters.
When comparing debt/GDP ratios of the US to countries that
have had hyperinflation, I believe one should include these excess
reserves that are earning interest. This makes the US debt/gdp ratio
even higher and further above the hyperinflation threshold.
Some people have noticed that this huge excess reserves is new and are rightly worried about what would happen if that money suddenly came into circulation. But the short term Treasury debt has the same danger. There is far more short term debt and far less long term debt in private hands these days. In part this is because the Fed has bought up so much of the long term debt with Operation Twist. The total short term debt from both the Treasury and the Fed is what can flood onto the market and start hyperinflation.
There are those that view bonds and excess reserves as part of the money supply. I think it is simpler to just view bonds and excess reserves as government debt and not part of the money supply but this view is valid as well. However, if you view things this way then you must recognize that the "velocity of money" for bonds and excess reserves is far far lower. As people move from those to cash it increases the velocity of money and so prices go up. When viewed this way, hyperinflation more due to the increasing velocity of money. But either way you view it, when governments that can print money have too much debt and then people don't want to hold that debt, you get hyperinflation.