Trying out Ubuntu Intrepid, I have discovered that RightAlt+F1 doesn't take me to tty13.
I've exhaustively tried every option presented to me in
dpkg-reconfigure console-setup, the best I've been able to manage thus far has been to have both Alt keys behave the same (LeftAlt+F1 and RightAlt+F1 both going to tty1).
Please note that using GNU
screen is not considered a valid response to the question - this is specifically about how to get the keyboard to behave properly with multiple consoles under Ubuntu. The crazy thing is that every other distro I've tried just handles this correctly, including Debian Lenny.