I'm not sure why you are getting an exception in the Shieldbase dll, but you need to specify a pin for the Chip Select.
http://www.netduino....o/schematic.pdf
If you're using a device connected to GoSocket 3 you would use (Cpu.Pin)4 as can be seen in the above schematic.
Also, I and others have run into SPI bugs when running under ~3000 kHz. So you may want to up your speed.