HID, SPI and the Arduino

I'm building a project that uses an HID OEM75 as the access point for a locking mechanism. We're doing our control through an Arduino Duemilanove (ATmega328). We're working with SPI for the security and (apparently) support. (I realize that any RFID chip has pretty weak security).

I'm currently struggling with receiving bytes via SPI from the card reader.

The card reader is in autonomous read mode, meaning it reads a card, sends a signal to the arduino (via a card present line, separate from SPI), which tells the arduino to send pull the Slave Select line high to activate the transfer cycle from the card reader. This is where I run into trouble, I can't understand how I can get the arduino to simply read the bytes coming in from the card reader without sending any command bytes to the card reader.

The default command structure, SPI.transfer(0x00), sends a byte (in this case the dummy byte 0x00) and then accepts a byte from the source, but because our source is operating autonomously, it won't accept the dummy byte (and that will actually mess up its operation).

To word it simply: how can we accept a string of bytes from the slave source without sending bytes from the Master Source, using arduino's SPI library?

You can leave the MOSI line disconnected if you never need to send data. The SPI bus needs data to be sent, because the hardware implementation is usually a shift register in the master and another in the slave, then a clock is used to send and receive each bit.