Today, WMAP announced results from three years of data collection, after an additional year of data analysis. The new information deals with "polarization" of the microwave background. Polarization of light indicates that the light bounced off of something. This is why polarized sunglasses are so nice -- they cut down on the polarized light glare from sunlight reflecting off the trunk of the car in front of you.
In the case of the cosmic microwaves, the polarization indicates that the light bounced off of electrons in the early universe. Right after the Big Bang, the whole universe was a very hot, glowing mass of atoms. After the Universe expanded for about 400,000 years, this soup of atoms cooled off enough to release the light we see now as microwaves. At some point, the first stars and quasars formed, releasing energy that warmed many of the atoms back up. We call this event "reionization," because the light energy from these stars and quasars removed electrons from the atoms in deep space for the second time in the history of the universe. These electrons, freed from their atoms, were now capable of reflecting the cosmic microwave background and polarizing it. And now, over 13 billion years later, WMAP has been able to detect the signals of reionization.
These signals tell us that the first stars in the universe formed 400 million years after the Big Bang (or 13.4 billion years ago). For those of you who understand the concept of "redshift," the redshift of reionization was between 7.3 and 12.1, with a best estimate of 9.3.
Anyway, we now know with much more certainty than ever before when the first stars in the Universe formed. Congratulations to the WMAP team for the results of their extreme efforts in analyzing data!