Comment 3 for bug 771246

I tried your xmodmaprc on a MBP6,2 with Natty and when I ran xmodmap in a console (as opposed to letting Gnome run it for me on login), it complained about removing Alt_R and Meta_R. Once I removed those, leaving:

remove mod1 = Alt_L Meta_L

 and it loaded, the keys were correctly swapped and I am able to use Alt-1 (i.e Cmd-1) to switch tabs in gnome-terminal. It might seem obvious, but did you check for xmodmap errors by running it manually first? Also, did you verify with xev that the correct keycodes are mapped to the correct symbols, as they should be? Does the non-responsive Alt problem occur in other applications besides gnome-terminal?