I tried that accidently at work today...
we must be behind a firewall, and I was trying to plug in a Static IP. The IP was not working, so the registration cancelled.
The internet access was completely down on the iMac I was installing it on, so I went back to the gateway & virtual network that we normally use. I could get on the internet after that, but the Apple Software Update was completely screwed up. I could not get it to update any software. I had to download the update to 10.0.4 from the apple website (which I wanted to do anyway), and tried to install that to see if it would fix the problem. I didn't!!!
I finally reinstalled the OSX for the third time, and it is working fine now.
Maybe Apple just wants to log how many installs we do. "Hey, there is John Doe installing it again! Wonder what he did this time..."