I had been running Ubuntu 5.10 through VMWare Player and everything was peachy keen. Then when 6.06 came out I upgraded and the networking quit working. When I went back to the original ‘clean’ 5.10 VM, the networking worked again.
I downloaded several different 6.06 VMs, and they all had the same problem. No networking.
Finally after reading the thread Ubuntu 6.06 – no networking in Server, works perfectly in Player, Works I was able to figure out the problem.
To fix it, Look in System->Administration->Networking and see what Ethernet interface it shows (eth0 or eth1). Then open up the file /etc/iftab and see if there is an entry in there. In my case the Network Settings screen was referring to eth1 while /etc/iftab referred to eth0. Apparently this mismatch is a problem. To fix it I just commented out the entry in /etc/iftab by putting a # in column 1 and saving the file.
I rebooted and voila! My networking works!