Having recently moved my desktop to Fedora 15 I am now using Gnome Shell (more on this later). However, as with many of the configuration settings for Gnome, having focus follow mouse is no longer something you can set using installed graphical programs.
Fortunately, a one line command can rectify this for those of us who have become accustomed to using focus follows mouse (if you’ve never tried out focus follows, you should give it a shot for a week or so - you might be suprised out how nice it is).
To use focus follows mouse, run the command:
gconftool-2 --set --type string /apps/metacity/general/focus_mode mouse
If you were trying out focus follows mouse, and wish to go back to clicking to focus on a window, you can run the command:
gconftool-2 --set --type string /apps/metacity/general/focus_mode click