The French newspaper Les Echos contains an interview with Christophe Ramboz of Vivendi Universal according to MCV-Online. He states that about one third of the source code was stolen. One of the consequences: the release of Half-Life has to be delayed at least 4 months (April 2004) to address potential problems caused by the theft according to the President of VU Interactive Publishing Intl. Stories on the same topic can be found here and here as well. No official word from Valve on this yet.
Update: Asked about the April release date Valve's Doug Lombardi states:
We are still assessing the effects of the attack. At this time, we have no further details we can share regarding the theft nor it's impact on the release of Half-Life 2.This "hum"-response was sent to the guys over at HalfLife.org.