Arctic sea ice has an average volume of 28,600 cubic kilometres at the end of April, the freeze. It has an average volume of 14,400 at the end of September, the thaw. (note: this is the average over the satellite period, and it is likely that the averages were at least slightly higher immediately prior to this period).
Since the satellite record began, Arctic sea ice volume has declined at a rate of 340 cubic kilometres per year.
From 2002 to the start of 2010, sea ice volume declined at a rate of 800 cubic kilometres per year. At that rate, by the end of this year sea ice volume should be 7,900 cubic kilometres below the long-term average. Over the longer term, that would mean an end to summer Arctic sea ice by 2020 (more than 14,400 cubic kilometres of volume loss).
However, at present, sea ice volume is 9,500 cubic kilometres below the long-term average. If we assume that it stays there until the end of the year, we will have had three year's worth of decline in a single year.
I believe that my prediction for 2014 as the year we first get an ice-free summer is a pretty good one. It should be noted that the scientist who works most closely with the direct data (which is partly gathered from submarines from the US Navy using sonar to scan the ice from beneath) predicted an ice-free summer for 2013 way back in 2003. His name is Dr Wieslaw Maslowski.
A link to the page with my bet with Willis Eschenbach: