Neurons are already optimized to make intelligence out of; transistors are not. Also, we already have an example of how to put them together the right way, which we can draw inspiration from in our own designs. Some subsystems of the brain are simpler than others, so we will very likely be able to simulate those before we have WBE.
Here is a short new publication from the Singularity Institute, on the 2-day workshop that followed Singularity Summit 2011.
Note the new publication design. We are currently porting our earlier publications to this template, too.