This is annoying when using applications like irssi, which have their own plans for alt+1. When you start up Terminal there are no open tabs and therefore Alt+1 does not accomplish anything, but Terminal still reserves it for it's own use. It would be nice if alt+1 became reserved only when there are tabs open.
You can change the shortcuts in the preferences dialog.
Resolving to INVALID, not a bug.
Sorry for posting to a closed bug, but: Are you sure this is invalid? In gnome terminal / konsole I can use Alt+1 when there is only one tab open. Assigning a different shortcut isn't a solution.