I also noticed this on ESXi (vSphere) 5.1. The machine serial number of my Mac mini isn't properly passed to the VM Guest.
Instead of getting the hardware serial number it shows up as VMWVk1dLteX2m....(etc.)
Although this is the Fusion forum, the serial number issue is not isolated to Fusion - other VMware products that can run on Macs have this issue too.
Hope this helps!
Colin