I had the same problem. Updated my 64bit and switched off for the night, next morning nothing.
Tried hard for a couple of hours. A connection was active to the router, a ping to the router was successful but no connection any further. Tried with no firewall, etc etc. Eventually created another user(demo), logged in as demo, setup the network connection and it worked straight away. The same connection, same address, eth0. Logged out and then back in to my account and all was again working.
Asus M4A79XTD EVO motherboard, AMD Phenom II X4 965 CPU, 4 GB ram.
Gigabyte NVIDIA 9800GT Silent graphics card, Dell FP2007 22 Digital Flat panel.
Samsung harddisks 2 x 200gb SP2004S,1 x Hitachi 750gb HDT 72107,Optiarc SATA DVD-RW. Sabayon 64 & 32bit.