Are you ******* kidding me? Thanks for $1200 down the drain. That was a working spi flash part off a laptop motorboat for, but hey, maybe some existing shields might trigger on 3.3v...might...why not either level shift the whole thing and say spi is 3.3v in the old location, and add a new one (oh I'm sure it will cost a few more pennies for that)...and then you get your maybe compatibility, without having people who've never used spi on the due before connecting their 3.3v parts (that's WHY I'm using the due is its 3.3v nature), to your 5v vcc line in the same header. Seriously, that he got to be one of the most stupid decisions I've ever seen. Yes...I'm pretty frustrated right now.
SPI header is standard on all Arduinos so it must have a standard power voltage (Always refer to Graynomad pinout diagram) , even though the data pins are only 3.3V on the Due.
That's all well and good, but it would have made sense to silkscreen it on the board, add a second spi header, he'll, maybe even level shift the logic in the 5v lines. After all, unless there's a diode on miso or something else, you're asking to burn out the due with those shields. So mah e if it's write only you bought backwards compatibility.
Reguardless, do something stupid like put a 5v vcc next to 3.3v logic in the same header, you should at the very least silkscreen it on the board so people don't fry $1200 of hardware for write only backwards compatibility.