@johanzebin: I don't know about this. The information would be great to add to the article though, so please let us kn ow how you get on. I think your best approach is to stick a frequency meter or oscilloscope on the PWM pin and look at what happens when you run the SPI code. You could write a little loop which writes or reads from the card and display the timings with and without tweaking the PWM rate. Experiment, have fun 
@jluciani: thanks! I looked into putting this up in the playground, but I don't have the time to learn wikispeak right now 
Any volunteers?? 