Update: Drivers are included in vSphere 4.1.

Download the ISO with drivers from HERE.

Burn to a CD and insert in the drive. Logon to the console as root and mount:
mount /dev/cdrom /mnt
Browse to /mnt and find the file “INT-intel-lad-ddk-igb-1.3.19.12.1-offline_bundle-185976.zip”

Install the driver:
esxupdate --bundle=INT-intel-lad-ddk-igb-1.3.19.12.1-offline_bundle-185976.zip update
Reboot.