As of January 1, 2011, the number of unused IPv4 addresses stands at 495.66 million. Exactly a year earlier, the number of available addresses was 721.06 million. So we collectively used up 225.4 million addresses in 2010. 242 million, really, if we ignore the unusual circumstance that Interop gave back nearly 17 million addresses.

via <a href="http://arstechnica.com/tech-policy/news/2011/01/2010-in-ip-addresses-225-million-down-496-million-to-go.ars">2010 in IP addresses: 225 million down, 496 million to go</a>.