musix, an argentinian distro which has loads of music software, my personal favourite
ubuntu studio, a project using ubuntu base for music production
64studio, same thing but focusing on 64bit machines, also has i386 version
dyne:bolic, rastasoft software, very interesting multimedia live cd
all of these do midi very well, I use all of them for various musical and midi tasks
if your prepared to pay
studio to go, from the makers of rosegarden
check em out, you can always multiboot...