According to Kickstarter [1] the success rate was 43% in 2010 and 46% in 2011. Wikipedia [2] suggests the success rate so far in 2012 is 44%, but the source is no longer available.

I don't know if that's just the success rate of crossing the funding threshold or success rate of delivering the project as well.

[1] http://www.kickstarter.com/blog/2011-the-stats [2] http://en.wikipedia.org/wiki/Kickstarter