I think it’s a bug in Hardy that affects only a few users, but if it’s not and it happens with the next release, and I don’t remember how to fix it, make sure the last line in the Keyboard section of xorg.conf looks like this and it’ll “remember” that I want right Alt as Compose:

Section “InputDevice”
Identifier “Generic Keyboard”
Driver “kbd”
Option “XkbRules” “xorg”
Option “XkbModel” “pc105”
Option “XkbLayout” “us”
Option “XkbOptions” “compose:ralt”