I'm using Ubuntu 11.10 and sometimes Nautilus icons stop being displayed and it looks like this:
Is there a way to fix this without rebooting?
I had the same problem (Ubuntu 11.10 oneiric).
I installed GNOME Tweak Tool:
It showed up as "Advanced Settings" in the menu (Dash home). I ran the tool and I started playing with some options, most likely the one that did the trick was: Theme-> Buttons Have Icons.
Hope this helps.