All went well the machine rebooted and I was asked for a couple of keychain password permissions - a little unusual but this was a large OS upgrade so I still wasn't troubled. Then when I opened Chrome there was no Internet connection, so I opened Network Preferences.
Weird, my IP address was something like 169.254.53.17 whereas my home network runs on 10.1.1.x. So I renewed my DHCP lease and every time I got a different 169.254.x.x address. Then I noticed the subnet mask was incorrect 255.255.0.0 instead of 255.255.255.0. I then manually assigned the machine an IP address, subnet mask, router IP, and DNS and assigned the machine's Mac address to the manual IP address in the router. All seemed fine the router's DHCP table showed the MacMini was connected and the router could ping it and the MacMini could ping the router and other machines on the network. But, still no internet. I was baffled - how could the MacMini have all the network settings correctly assigned, be able to ping and respond to pings without being able to connect to the Internet. Time for sleep.
In the morning I went to Twitter and found several people had the same problem and one guy, Ralph Rottmann, had a solution. It seemed that the 10.7.3 update had messed up the firewall's permissions so it was rejecting all traffic apart from pings. Why this didn't occur on my two other Macs, but did on my main work horse, which is deliberately kept quite vanilla, I have no idea. I overwrote the authorization system configuration file with the same file from one of my working Macs, rebooted and bingo all back to normal!
The upgrade cost me an afternoon of lost work and several frustrating hours. I'm very surprised that such a major "bug" could slip out of Cupertino, surely many developers using the pre-public release encountered this, so why didn't Apple fix it?