Marked one of the above posts as the answer as I have a P9X79 Pro and this thread answers it for me. Looks like Asrock and Gigabyte are onto it too.
http://rog.asus.com/forum/showthread.php?26501-RIVE-3301-Patched-for-TRIM-in-RAID!
Thanks heaps Nodens! I haven't actually done it yet, I have a Samsung 830 128gb that I'm booting from and they aren't available in NZ any more, so I will need to buy two more SSD's that I can't quite justify doing this second!