I still see this bug in Ubuntu 10.04 and 10.10. Amazing.
I always have to use Bryan's workaround, from https://bugs.launchpad.net/ubuntu/+source/console-setup/+bug/76901.
"Selecting the "Alt and Meta are on the Alt keys" option in "Keyboard Preferences" fixes this behavior"
After that we can finally disable Right Alt as the 3rd level key and it works again as an ALT key.