Tried to upgrade to ESXi 5.5.0 today, and got an error that there was no usable network device in the computer.
Found that strange, since the current ESXi 5.0.0 works fine with the on-board network adapter.
A bit of searching and it seems that VMware didn’t include the drivers for Realtek network adapters on the install disk. (makes me wonder what else was left out).

After a bit of searching, came up with a link describing the problem, and another link on how to add the driver onto the install disk.