The IANA distributes addresses to the RIRs as
Each of these
/8 blocks holds 16,777,216 addresses.
There is an agreement in effect that once five blocks remain, each RIR gets one last block. At the current rate that will happen in 2011. At that point, APNIC and RIPE will have enough addresses to continue business as usual for about another year at current rates, ARIN and LACNIC for around three years, and AfriNIC for nearly seven years.
In a ceremony in Miami on Feb. 3, 2011, the final five blocks of IPv4 addresses were given out to the five Regional Internet Registries (RIRs) that further distribute IP addresses to ISPs. The five RIRs still have tens of millions of addresses as working inventory, but once those addresses are given out, it's over. (From a post on Ars Technica by Iljitsch van Beijnum.)