It couldn't hurt to at least try the 48GB first. If you're going to have to rebuild, the worst that'll happen is that you'll carry over the 48GB to the next build.
Good point. I have seen motherboards rated for less before simply because the concept of the bigger chips didn't exist when the board was originally released and couldn't be tested, but it still worked. if it worked for one guy with x58 Chipset, and you're prepared to upgrade if it doesn't work its at least worth a shot.
i'd like to update this - I installed 48GB (G.Skill Ripjaws DDR3 1333mhz) on my i7 920 EX58-UD5 machine today, and it is working fine! Windows recognises the RAM correctly and its working very well even not "officially" supported.