The pool for IPv4 addresses has been depleted. That means the protocol for the Internet as we know it has been used up, and the Internet must move toward IPv6, the next-generation protocol, which has much more room for growth. 

"IPv4 has approximately four billion IP addresses (the sequence of numbers assigned to each Internet-connected device)," the Internet Society recently explained, indicating that "the explosion in the number of people, devices and web services on the Internet" is the cause of the depletion. "IPv6, the next-generation Internet protocol, which provides over four billion times more space, will connect the billions of people not connected today and will help ensure the Internet can continue its current growth rate." 

"This is truly a major turning point in the on-going development of the Internet," said ICANN President and CEO Rod Beckstrom. "Nobody was caught off guard by this, the Internet technical community has been planning for IPv4 depletion for quite some time. But it means the adoption of IPv6 is now of paramount importance, since it will allow the Internet to continue its amazing growth and foster the global innovation we’ve all come to expect."

World IPv6 day had already been scheduled for June 8. This is a day in which major web properties like Google, Facebook, and Yahoo (the three of which make up a combined billion visits per day) join major content delivery networks like Akamai and Limelight Networks for a 24-hour global trial of IPv6, the next-generation Internet protocol. More on this here. Update: Bing has now announced participation in IPv6 day. 

The American Registry for Internet Numbers (ARIN) is publishing a daily report of its own IPv4 inventory of available addresses on its home page. It also has a helpful FAQ page set up.  The Number Resource Organization (NRO) has further explanation of the news.