Wallpaper stretching over multiple monitors on Fedora 19 (Gnome 3)

I have no idea why the configuration option is not available in the gnome control panel but you can set this using dconf-editor.

Run dconf-editor from a terminal, navigate to org.gnome.desktop.background -> picture options and set it to spanned:

Installing gnome-tweak-tool lets you customize not only the wallpaper alignment, but other absent options such as enabling / disabling minimize and maximize buttons, showing icons on the desktop, changing the shell theme and colors, changing fonts and its options (size, hinting, etc).