No solution yet, but I strongly believe I have got the same problem Path has.
When the computer has been off for a while (I don't know for how long it has to be off exactly, but overnight is a sure winner) the fan does not spin up immediately when pressing the power button. Sometimes, but not always, I can see it move just a little. Then, after about 10 seconds (when the BIOS scans the DVD-drives) the fan finally spins up.
This often results in a CPU-fan error message. Even then the fan does not spin at the RPM is it supposed to spin. It takes quite some time (over 10 minutes) before it spins at its correct RPM. Not very convenient if you are using Speedfan.
The funny thing is, that it does seem to spin up immediately when you power off the computer, and then power it on again after a few minutes.
I hope someone can help us.