The eastern span (e.g. between Oakland and the Yerba Buena Island tunnel) is an entirely new bridge. The western span (the suspension part leading into downtown SF) is unaffected.
I can see the bridge from my house.
The really sad thing is that the original bridge was built in 18 months in 1933, without computers.
This new bridge will not be completed until 2013, and it's been under construction over 3 years already.