I no longer use Gnome/Nautilus, so I'm not sure whether there's been any changes, but it used to be Nautilus>edit>preferences>behaviour?
I know it had me stumped a couple of times....
EDIT:
This just posted in another thread:
Code:
http://www.linuxquestions.org/questions/linux-hardware-18/fedora-9-core-single-click-acts-like-double-click.-684118/
hth,