I have this problem too.
I bought yesterday a Retina,15-inch, Early 2013, 2,7 GHz Intel Core i7, 16 GB 1600 MHz DDR3 (the new retina display macbook) and have an issue of sudden changes of the speed of the fans without apparent cause.
The fans go suddenly up to 5918 rpm without apparent cause, with the macbook idle. This is the second time it has happened and I am very worried about it. The last peak of rpms occurred when I was writing a letter on Pages. The CPU workload was negligible.
You can see the unexpected peaks of rpms in the graph. I was not working in any CPU intensive task.
Hope this is solved soon.