I've reproduced this error many times:
1) Download VMWare Server, and Install
2) Download Ubuntu Server 7.04, and install in VMWare
3) Complete the OS installation
4) Networking is functional, eth0 is present
5) Shutdown
6) Copy the virtual machine or rename folder that it's in
7) Launch new copy, VMWare prompts to create a new ID
Networking is no longer functional, eth0 is gone
The problem occurs whether or not you install VMWare tools