Can you read the state of the SDI-pin manually?
I'm thinking perhaps there's some other function enabled on that pin preventing it from being read properly. Though if analog functions are enabled the pin usually reads high, not low...
If you CAN read it manually then that's probably not the case. Are you probing directly at the input pin on the PIC itself and is that PIN actually the correct one? Or are you probing at the output pin of the accelerometer?

Which PIC are you using?
Which pins are you using?

/Henrik.