ARS Technica writes about the need to switch to IPv6. The current IPv4 address space offers only 3.7 billion possible addresses and with more than 2.7 billion IPv4 addresses currently in use it's only a matter of years until IPv4 will reach its limit. Current estimates point out this will likely happen sometime in 2011 or 2012.
So, how long will a billion quench our thirst for addresses? Geoff Huston at the Asia Pacific Network Information Centre has written a script that downloads the relevant information and creates daily predictions. The current ones target 10 February 2011 as the moment that IANA will give out the last of its blocks to one of the regional registries, and 17 December 2011 as the day that the last RIR will hand out the last IPv4 address to an ISP (or end-user).
Personally, I've always thought that Geoff's model is a bit pessimistic; in order to run out of addresses in a little over three years, we'd have to increase the rate at which addresses are handed out by 30 percent each year. In 2007, the increase over 2006 was 19 percent. The year before that there was no increase to speak of.