and its amazing, no more agc on my on board sound and I can totally disable it for use with an external mic (but loose regular mic input)
http://www.derkoi.co.uk/film/index.php?option=com_content&view=article&id=89:how-to-install-magic-lantern-firmware-on-the-canon-550d-t2i&catid=1:latest-news&Itemid=1
everything you need is right there! good luck! I'll post this same thread in the HDSLR filmmakers cult.