To scroll a single line in gnome-terminal you have to press Ctrl-Shift-↑ or Ctrl-Shift-↓ in Ubuntu.
In other distributions (e.g. Xubuntu) you only have to press Shift-↑ or Shift-↓ for the same behaviour. I would like to change the shortcuts in Ubuntu to work the same, but so far I haven't found an option or config file to change.
The reason why I would like to change this are two-fold: First, I find it more convenient (not having shift and ctrl above each other on a Thinkpad). Second, it is more consistent to the shortcuts for scrolling an entire page ( Shift-Pgup and Shift-Pgdown).
Also, I remember reading the reason behind using ctrl-shift instead of just shift, but I cannot locate that discussion anymore.