I'm using the sparkfun.com microDC shield with the SdFat.h library under Arduino 0023.
Thanks for including this detail.
I have noticed in the past that this shield seems to reset the Arduino when an SD card is inserted--I'm not sure why.
You could try contacting SparkFun tech support via email.
P.S. My *assumption* is that something is being shorted when the card is being inserted--perhaps related to a "card present" pin--in which case I'd be tempted to look for a trace to cut if SF tech support can't help.