Quoting from the Microsoft official download page:
The Virtual Machine Network Driver allows the Device emulator’s OS (or even the Virtual PC OS, as the case may be) to emulate its own network connection. Since the physical network interface on the host machine is now “virtualized”, you have a way to get two IP Addresses – one for the host PC, and one for the operating system that is running within the Device Emulator (or Virtual PC). Device Emulator users using the VMNet Driver can connect to the host machine over TCP or UDP as the alternative to the standard “Activesync over DMA” solution.
Thanks to Steven Bink for the news.