Under what circumstances did this occur?
I had this happen today on my Air. Problem is, the IP it said was taken, is reserved for my air. In fact, all my machines and devices have reserved IPs, so it would be impossible for the IP to be taken by another machine.
When this happened, I was opening my air after it had been asleep. I think this might be a bug that occurs when waking from sleep.
Let me know what was happening when this happened to you.