According to an article by USGS the earthquake in the Indian Ocean Dec 26 slowed the earth by 2,7 milliseconds! Here is a quote:
“Richard Gross at JPL has modeled the coseismic effect on the Earth’s rotation of the December 26 earthquake in Indonesia by using the PREM model for the elastic properties of the Earth and the Harvard centroid-moment tensor solution for the source properties of the earthquake. The result is:
change in length of day: -2.676 microseconds ”
Link to the article:
http://earthquake.usgs.gov/eqinthenews/2004/usslav/neic_slav_faq.html