Less Than Zero
When this is written, the yield has dropped further, to -0.06%.
What is going on here? Are investors crazy? Yes and no. Clearly, they have far too much faith in the fiscal situation of the U.S. government, who has both a budget deficit and a debt level that is somewhat larger than that of Portugal relative to GDP. U.S. government bonds definitely don't deserve the AAA-rating and general "safe haven"-status that have caused these low yields.
On the other hand, with the economy heading for a double dip recession, many alternative investments like stocks don't look very attractive either, so in part these low yields are understandable.
There are of course many countries whose government bonds look a lot more attractive as they have higher yields and/or a stronger fiscal situation, including for example Brazil, Australia and Sweden, but that is associated with exchange rate risks for investors that live outside these countries.