SD Card Interface with Pic Mcu - Part_1
You could try connecting a logic analyzer to the bus. This will allow you to double check your SPI bus timing and make sure that you are sending what you think you are sending from the PIC to the SD card.If you don't have a logic analyzer there are many bus analyzers on the market for ~$100 that work very well for debugging serial protocols like SPI and I2C.
http://www.usbee.com/busbee.html

http://www.saleae.com/logic/