Comment 9 for bug 773734

Replying to myself again... I found one way to get "usable" dual screen in Natty:

1.- Boot normally without disabling eDP1 (i.e. forget about comment #8)
2.- Login using the Gnome Classic interface (yes apparently Unity is causing some problems here)
3.- Manually disable the eDP1 output, e.g. with xrandr, and only enable the LVDS and VGA output. This line works for me:
    $ xrandr --output eDP1 --off --output --LVDS1 --mode 1366x768 --output VGA1 --mode 1680x1050 --primary
4.- Yay, both screens work at the same time at their max resolutions!

This indicates that there is probably more than one bug at play here, and the fact that Unity and Gnome-shell have different results could be related to bug 661450, or more specifically to bug 726892, which is a duplicate of it.

Hope this will help those looking for a solution...