EDIT : this should be in the technical help forum, sorry. (I tried to move it but the forum won't allow me to delete this thread)

Sorry to all of you : I don't have the solution. BUT WE could obtain it from Ubisoft : we must ask for a patch ! Just go to

http://ubisoft.custhelp.com/cgi-bin/ubisoft.cfg/php/enduser/ask.php

then log-in and you'll be on the correct page to submit this kind of request. I bought this game the day it came out but I simply can't stand this mouse inversion problem ! I used to wait for a patch... but it never came out... So, this is the only way. PLEASE DO USE THE LINK ABOVE AND REPORT THE PROBLEM.
Thanks

N.B: GRRRRRRRRRRREAT GAME BTW !!!