I now have two nice icons on my gnome panel (a "normal" screen and an upright screen) with two tiny scripts behind them. To turn the screen upright:
and to turn the screen to "normal" position:
xrandr -o left
gconftool-2 -s /desktop/gnome/font_rendering/rgba_order -t string "vrgb"
If someone wants to use the icons, I attached them. They could be more fancyful (with some arrows showing they are for rotation), but they look good enough. (I slightly edited the icon I found at /usr/share/icons/hicolor/scalable/apps/gnome-display-properties.svg.)
xrandr -o normal
gconftool-2 -s /desktop/gnome/font_rendering/rgba_order -t string "rgb"