I understand that the pins are not arbitrary, but is there any reason why we shouldn't re-map them to the corresponding UNO header pinout as this discussion suggests:
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1286873309
Note: I just noticed that I wrote the wrong pin assignments above: 50, 51, 52, 53 on Mega corresponds to 12, 11, 13, 10 on UNO.
This would ensure that Duemilanove and UNO shields would be pin-compatible. There just seems to be a lot of shields out there that expect SPI on pins D11-13 still. Of course we will still have SPI on the ICSP header.