Intel PM55 Chipset to be Upgraded to B3 Stepping
 

Just like its desktop-bound brother, the PM55 mobile chip is set to see the transition from the B2 to B3 stepping in near future. Intel’s B3 stepping will have a new MM number and S-spec number, a processor MRC/microcode update, and a storage driver upgrade from MSM 8.9 to RST 9.5. There are no motherboard modifications needed, the B3 chip will be supported by updated firmware and BIOS.

http://en.expreview.com/2009/11/23/i...-stepping.html