We have the same issue.  3 out of 11 Vostro 460 missing NIC. I totally agree with jason629. Either call Dell for a new MB or get a new NIC. I spent hours and hours trying to update bios/driver, restart, unplug, reset MB, shaking. I ended up geting  new NICs. Disappointed. Dell's quailty is going downhill.  
I have 10 Vostro 460s, the network cards are not being found on startup.  You have to reboot the systems 3-4 times to get Windows to find the Network cards.  All drivers are up to date.  This may be related to a Windows update, since the machine were fine for a month or so until an Update came out.  
Vostro 460 Network card not found Code 31 need to reboot 3 to 4 times