Germany tech site Golem reports NVIDIA CEO Jen-Hsun Huang openly talked about what went wrong during the development of Fermi at the company's GPU Technology Conference in San Jose. A Google translated version of the article can be read over here.
Basically, what Huang claims is that the whole fabric had to be redesigned because the first A0 stepping of Fermi was completely broken. NVIDIA's CEO says technical as well as management errors are to blame, he revealed that his architecture engineers and those dealing with the physics involved are sitting in two different departments, suggesting that these teams didn't communicate properly.
The simulations showed Fermi should have worked, but in practise the first design completely failed so NVIDIA had to rush out a complete redesign in half a year time to get it to work.