What change would like to see?

An SPI library.

Why?

To provide a standard API for users and library writers.

Would this cause any incompatibilities with previous versions? If so, how can these be mitigated?

No.

Just for reference, here's a link to an SPI library on the playground: http://www.arduino.cc/playground/Code/Spi

FWIW, for my own purposes I implemented a slightly different public interface and then subclassed it for my actual device:

class SpiDevice { public: SpiDevice();

void begin(); // Uses default (pin 10)
void begin(byte selectPin);

// May make these private (or protected) in the final library?
void deselect();
void select();

byte transfer(volatile byte data);
void transfer_bulk(const uint8_t* srcptr, unsigned long int length);

};

Added as part of a rewrite of the Ethernet library by Christian Maglie (r1023).