When Intel debuted the Centrino mobile platform solution last March, newly launched Centrino notebooks featured an 802.11b WLAN module. The company introduced 802.11a/b modules for the Centrino platform in the third quarter.Source: DigiTimes
According to Wu, Intel plans to launch 802.11a/b/g modules in mid-2004, and the modules will continue to use Intel’s second-generation Calexico chipset.
Intel: "Volume production of 802.11g modules to start end of this month"

The volume production of 802.11g modules for Centrino laptops is going to start at the end of this month. These will be used in Intel his second generation wireless chipset, which is called Calexico.