IPv4 was designed in the 1970's and supports just over 4 billion unique addresses. Back then, nobody could ever have imagined the internet becoming what it is today.
As of 1 February 2011, the global pool of IPv4 addresses has been depleted, The first regional pool ran out on 15 April 2011, with the remaining regional pools slated to run out in late 2011/early 2012 (Europe and North America) and continuing until 2013/4 (Latin America and Africa). Individual ISPs and hosting companies should have between three and twelve months after their regional pool is empty. By 2013, it will be hard/expensive to get a new IPv4 allocation outside of Africa and Latin America.
In the early 1990's people started to realise that we were going to run out of IP addresses and a taskforce was developed to decide on a new protocol. The protocol that was settled on was IPv6.
There is an excellent talk from DefCon 18 on youtube that explains a lot of the history around IPv6. You can find Part 1 here.
